Solid-state image pickup apparatus, signal processing method for a solid-state image pickup apparatus, and electronic apparatus

ABSTRACT

A solid-state imaging device includes a pixel array section that has at least one pixel with a photoelectric conversion unit and a charge detection unit. A driving section is configured to read out a signal of the pixel, a first portion of said signal being based on signal charge, a second portion of said signal being based on a reset potential. A signal processing section is configured to read out the first portion of the signal as a reference voltage, with the reference voltage being adjusted to cause the first and second portions of the signal to be within an input voltage range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 14/927,212,filed Oct. 29, 2015, which is a Continuation of application Ser. No.14/703,286, filed May 4, 2015, now U.S. Pat. No. 9,215,390, issued Dec.15, 2015, which is a Continuation of application Ser. No. 14/610,086,filed Jan. 30, 2015, now Abandoned, which is a Continuation ofapplication Ser. No. 14/046,268, filed Oct. 4, 2013, now U.S. Pat. No.8,964,086, issued Feb. 24, 2015, which is a Continuation of applicationSer. No. 13/064,338, filed Mar. 21, 2011, now U.S. Pat. No. 8,576,317,issued Nov. 5, 2013, which claims priority to Japanese Priority PatentApplications JP 2011-015994 filed in the Japan Patent Office on Jan. 28,2011 and JP 2010-079017 filed in the Japan Patent Office On Mar. 30,2010, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to a solid-state image pickup apparatus, a signalprocessing method for a solid-state image pickup apparatus, and anelectronic apparatus.

2. Description of the Related Art

A solid-state image pickup apparatus is known which includes a unitpixel which in turn includes a photoelectric conversion section, acharge detection section and a transfer gate section for transferringcharge accumulated in the photoelectric conversion section to the chargedetection section. A solid-state image pickup apparatus of the typedescribed usually carries out a noise removing process by correlationdouble sampling in order to remove noise upon a resetting operation. Inthe following description, the charge detection section is referred toas FD (Floating Diffusion) section. As a method of the noise removingprocess, a method which uses digital signal processing and anothermethod which uses analog signal processing are available.

A solid-state image pickup apparatus which uses digital signalprocessing as a noise removing process is known and disclosed, forexample, in Japanese Patent Laid-Open No. 2006-340044 (hereinafterreferred to as Patent Document 1). The solid-state image pickupapparatus disclosed in Patent Document 1 incorporates column-parallelADCs (Analog-Digital Converters; analog-digital conversion circuits) andis configured such that a plurality of unit pixels are arrayed in rowsand columns and an ADC is disposed for each column.

In the solid-state image pickup apparatus incorporating column-parallelADCs, a reset level V_(rst) read out first is set as a reference voltageV_(zr) for an AD conversion circuit, and the reference voltage V_(zr) isused to AD convert the reset level V_(rst) and a signal level V_(sig).In particular, the reference voltage V_(zr) is made equal to the resetlevel V_(rst) so that, even if the reset level V_(rst) is fluctuated bynoise, the output amplitude |V_(sig)−V_(rst)| of the pixel by signalcharge can be included stably in an input voltage range of the ADconversion circuit.

Generally, the reference voltage V_(zr) of the AD conversion circuit canbe adjusted only within a sufficiently small range with respect to theinput voltage range of the AD conversion circuit. Accordingly, thereference voltage V_(zr) for the AD conversion circuit preferably is asignal whose fluctuation width is stable and limited like the resetlevel V_(rst). On the contrary, a signal whose voltage exhibits a greatamplitude in response to an incident light amount like the signal levelV_(sig) is not suitable as the reference voltage V_(zr) for the ADconversion circuit.

A noise removing process of an existing solid-state image pickupapparatus is premised on an assumption that the reset level V_(rst) isread out first and the signal level V_(sig) is read out immediatelysucceeding the reset level V_(rst) as in the solid-state image pickupapparatus which incorporates column-parallel ADCs described above. Onthe other hand, a solid-state image pickup apparatus which cannot readout the reset level V_(rst) immediately preceding to the signal levelV_(sig) cannot acquire, before the signal level V_(sig) of a certainunit pixel is AD converted, the reference voltage V_(zr) from the samepixel.

As a solid-state image pickup apparatus which cannot read out the resetlevel V_(rst) immediately preceding to the signal level V_(sig), acomplementary metal oxide semiconductor (CMOS) image sensor having aglobal exposure function is known and disclosed, for example, inJapanese Patent Laid-Open No. 2001-238132 (hereinafter referredhereinafter referred to as Patent Document 2). In the CMOS image sensor,in order to implement simultaneous exposure of all pixels, chargegenerated in the photoelectric conversion section is transferred to theFD section simultaneously with regard to all pixels. Then, in the statein which the signal charge is retained in the FD section, a reading outoperation of the signal charge is carried out successively for allpixels.

As another solid-state image pickup apparatus which cannot read out thereset level V_(rst) immediately preceding to the signal level V_(sig),another CMOS image sensor is known and disclosed in Japanese PatentLaid-Open No. 2009-020176 (hereinafter referred to as Patent Document3). The solid-state image pickup apparatus disclosed in Patent Document3 includes a memory section for retaining photocharge transferredthereto from the photoelectric conversion section separately from thecharge detection section. Also a CMOS image sensor wherein photochargegenerated by a PN junction is read out directly by an amplificationtransistor is known and disclosed, for example, in “128×128 CMOSPHOTODIODE-TYPE ACTIVE PIXEL, SENSOR WITH ON-CHIP TIMING CONTROL ANDSIGNAL CHAINELECTRONICS,” SPIE, vol. 2415, Charge-Coupled Devices andSolid State Optical Sensors V, paper no. 34 (1995) (hereinafter referredto as Non-Patent Document 1). Further, an image sensor which uses anorganic photoelectric conversion film is known and disclosed, forexample, in Japanese Patent Laid-Open No. 2008-228265 (hereinafterreferred to as Patent Document 4).

In a solid-state image pickup apparatus, the FD section is reset onceupon simultaneous transfer from all pixels or upon starting of exposure,and therefore, at a timing at which a signal is to be read out, signalcharge is accumulated or retained already in the FD section. Therefore,in order to remove fixed pattern noise such as a threshold valuedispersion of an amplification transistor and so forth, it is necessaryto set, after the signal level V_(sig) is read out, the FD section to apredetermined potential and read out the predetermined potential as thereset level V_(rst) as seen in FIG. 44.

However, in a solid-state image pickup apparatus wherein signal readingout is executed in a state in which signal charge is retained in the FDsection for simultaneous exposure of all pixels or in a solid-stateimage pickup apparatus wherein signal charge is accumulated directly inthe FD section and signal reading out is carried out, the FD sectioncannot be set to a predetermined potential immediate before the signallevel V_(sig) is read out. In this instance, a reference voltage to beused for AD conversion of the signal level cannot be acquired.Therefore, a predetermined voltage is generated by external applicationor by means of a resistor array or the like and supplied as a referencevoltage to the AD conversion circuit as disclosed, for example, inJapanese Patent Laid-Open No. 2006-020176 (hereinafter referred to asPatent Document 5).

SUMMARY OF THE INVENTION

However, the reset level suffers not only from a dispersion amongdifferent unit pixels arising from a threshold dispersion of anamplification transistor and so forth but also from a difference by agreat spatial fluctuation of a characteristic in a plane in atwo-dimensional array of the unit pixels, that is, by an in-planedistribution or from a time-dependent variation by the temperaturevariation upon operation. Accordingly, it is necessary to assure asufficient margin to the reset level. Therefore, it is necessary toexpand the convertible input voltage range of the AD conversion circuitto a voltage range determined by adding such variation factors to apixel output amplitude of signal charge to be acquired actually.

FIG. 45 illustrates an example of an in-plane distribution of the resetlevel in a longitudinal direction, that is, in a column direction whichis an array direction of pixels in a pixel column. In addition to adifference of the reset level between adjacent pixels, a greatfluctuation in characteristic in a plane, that is, an in-planedistribution, is observed. In the case where a fixed voltage is appliedas a reference voltage as seen in FIG. 46, since a great error appearsdepending upon a pixel, it is necessary to expand the input voltagerange of the AD conversion circuit.

For example, FIG. 47 illustrates a reset level with respect to areference voltage and a signal level at a maximum amplitude in the casewhere the signal amplitude is −1 V in the maximum. In this instance,while the Peak-to-Peak signal amplitude from the reset level to thesignal level is 1 V, an AD-convertible input voltage range ofapproximately 2 V is required. Further, in the case where the resetlevel is fluctuated upwardly or downwardly by a temperature variation,since the reference voltage is fixed, it is necessary to further expandthe input voltage range as seen in FIG. 48.

In the related art disclosed in Patent Document 5 wherein apredetermined voltage is generated by external application or by meansof a resistor array or the like and supplied as a reference voltage toan AD conversion circuit, the correlation between the reference voltageand the reset level is low. Therefore, the related art has a problemthat the AD convertible input voltage range is small. The related artfurther has a problem that the AD convertible input voltage range isdecreased also by the in-plane distribution of the pixel dispersion andthe temperature dependency of the reset level.

It is to be noted here that, while setting of a reference voltage for anAD conversion circuit in a solid-state image pickup apparatus wherein anoise removing process is executed by digital signal processing isdescribed above, the problems described above are not limited to thecase of digital signal processing. In other words, the problemsdescribed similarly apply also to a solid-state image pickup apparatuswherein a reference voltage is used to carry out signal processing foran analog signal from a unit pixel (details are hereinafter described).

Therefore, it is desirable to provide a solid-state image pickupapparatus, a signal processing method for a solid-state image pickupapparatus and an electronic apparatus having a solid-state image pickupapparatus which can effectively eliminate a great difference in in-planefluctuation of a characteristic and an offset component which dependsupon the magnitude of parasitic capacitance.

According to an embodiment of the present invention, there is asolid-state image pickup apparatus, including:

a pixel array section in which a unit pixel including a photoelectricconversion section and a charge detection section for detecting chargegenerated by photoelectric conversion by the photoelectric conversionsection is disposed;

a driving section adapted to carry out driving of reading out a signalof the unit pixel divisionally by twice as a first signal and a secondsignal; and

a signal processing section adapted to set the first signal read outfirst from the unit pixel as a reference voltage for a processable inputvoltage range of the signal processing section, adjust the referencevoltage so that the first and second signals may be included in theinput voltage range and carry out signal processing for the first andsecond signals using the adjusted reference voltage.

According to another embodiment of the present invention, there is asignal processing method for a solid-state image pickup apparatus inwhich a unit pixel including a photoelectric conversion section and acharge detection section for detecting charge generated by photoelectricconversion by the photoelectric conversion section is disposed, thesignal processing method including the steps, carried out by a signalprocessing section, of:

reading out a signal of the unit pixel divisionally by twice as a firstsignal and a second signal;

setting the first signal read out first from the unit pixel as areference voltage for a processable input voltage range of the signalprocessing section;

adjusting the reference voltage so that the first and second signals maybe included in the input voltage range; and

carrying out signal processing for the first and second signals usingthe adjusted reference voltage.

According to further embodiment of the present invention, there is anelectronic apparatus, including:

a solid-state image pickup apparatus including

-   -   a pixel array section in which a unit pixel including a        photoelectric conversion section and a charge detection section        for detecting charge generated by photoelectric conversion by        the photoelectric conversion section is disposed;    -   a driving section adapted to carry out driving of reading out a        signal of the unit pixel divisionally by twice as a first signal        and a second signal; and    -   a signal processing section adapted to set the first signal read        out first from the unit pixel as a reference voltage for a        processable input voltage range of the signal processing        section, adjust the reference voltage so that the first and        second signals may be included in the input voltage range and        carry out signal processing for the first and second signals        using the adjusted reference voltage.

In the solid-state image pickup apparatus, the first signal read outfirst from the unit pixel is used as the reference voltage for the inputvoltage range within which the input voltage can be processed by thesignal processing section. At this time, if the first signal is a signalbased on signal charge accumulated or retained in the charge detectionsection, that is, a signal level, then the signal level is used as thereference voltage. On the other hand, if the first signal is a signalbased on the reset voltage when the charge detection section is reset,that is, a reset level, then the reset level is used as the referencevoltage.

After the first signal read out first is set as the reference voltage,the reference voltage is adjusted so that the first and second signalsmay be included in the input voltage range within which the inputvoltage can be processed by the signal processing section. As a resultof the adjustment of the reference voltage, signal processing can beexecuted for the first and second signals in the state in which thefirst and second signals are included in the input voltage range.Accordingly, irrespective of whether the first signal read out first isthe signal level or the reset level, the signal processing can becarried out with certainty for the first and second signals after thereference voltage is set using the signal of the pixel of the processingobject.

With the solid-state image pickup apparatus, since the voltage is setusing a signal of a pixel of a processing object, such a greatdifference in in-plane fluctuation of a characteristic or an in-planedistribution and an offset component which depends upon the magnitude ofparasitic capacitance as in the case in which a predetermined voltagegenerated separately is used to set a reference voltage can beeliminated effectively. Consequently, a margin to an output amplitude ofthe pixel necessary for the input voltage range within which the inputvoltage can be processed by the signal processing section can bereduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram showing a general configuration of aCMOS image sensor according to a first embodiment of the presentdisclosure;

FIG. 2 is a circuit diagram showing an example of a configuration of aunit pixel;

FIG. 3 is a timing waveform diagram illustrating a correlation doublesampling operation in the case where a reset level and a signal levelare read out in this order;

FIG. 4 is a timing waveform diagram illustrating operation in the casewhere a reset level read out first is set as a reference voltage for anAD conversion circuit;

FIG. 5A is a circuit diagram showing a example of an existingconfiguration of a comparator which configures an input stage of an ADconversion circuit and FIG. 5B is a diagrammatic view illustrating anoperation range of the comparator;

FIG. 6 is a diagrammatic view illustrating a voltage of an internal nodeand an operation range of the comparator of FIG. 5A upon CDS driving;

FIG. 7 is a similar view but illustrating a voltage of the internal nodeand an operation range of the comparator of FIG. 5A in a highilluminance state upon DDS driving;

FIG. 8 is a similar view but illustrating a voltage of the internal nodeand an operation range of the comparator of FIG. 5A in a low illuminancestate upon DDS driving;

FIGS. 9A and 9B are diagrammatic views illustrating an operation rangeof the comparator of FIG. 5A in a low illuminance state and a highilluminance state, respectively, in the case where the inclination of areference signal is inverted;

FIG. 10A is a circuit diagram showing an example of a configuration of acomparator of a PMOS input configuration and FIG. 10B is a diagrammaticview illustrating an operation range of the comparator;

FIG. 11A is a circuit diagram showing an example of a configuration of acomparator according to a working example 1 which includes a function ofadjusting a reference voltage for an AD conversion circuit and FIG. 11Bis a diagrammatic view illustrating an operation range of thecomparator;

FIG. 12 is a waveform diagram illustrating a driving waveform in a lowluminance state upon DDS driving in the case where DDS driving and CDSdriving are carried out alternately;

FIG. 13 is a similar view but illustrating a driving waveform in a highluminance state upon DDS driving in the case where DDS driving and CDSdriving are carried out alternately;

FIG. 14A is a circuit diagram showing an example of a configuration ofthe comparator according to the working example 1 in the case where aninitial voltage setting circuit is configured using a PMOS transistorand FIG. 14B is a diagrammatic view illustrating an operation range ofthe comparator;

FIG. 15 is a timing waveform diagram illustrating circuit operation ofthe comparator according to the working example 1 of the firstembodiment of the present disclosure;

FIG. 16 is a circuit diagram showing an example of a configuration of acomparator according to a working example 2 which includes a function ofadjusting internal reference voltages of an AD conversion circuit;

FIG. 17 is a timing waveform diagram illustrating waveforms of controlsignals, a reference signal of a ramp waveform and a signal voltage inthe comparator according to the working example 2;

FIGS. 18A to 18D are schematic views illustrating a channel length andan injection amount and illustrating working effects of the comparatoraccording to the working example 2;

FIG. 19 is a circuit diagram showing an example of a configuration of acomparator according to a working example 3 which includes a storagecircuit section on a succeeding stage to a comparison circuit section;

FIG. 20 is a timing waveform diagram illustrating operation of a firstdriving example of the comparator according to the working example 3;

FIG. 21 is a timing waveform diagram illustrating operation of a seconddriving example of the comparator according to the working example 3;

FIG. 22 is a similar view but illustrating driving in the case whereslopes of reference signals in CDS driving and DDS driving have a samegradient;

FIG. 23 is a similar view but illustrating driving in the case where aslope of a reference signal on the DDS driving side is set to a gradientsteeper than that on the CDS driving side;

FIG. 24 is a timing waveform diagram illustrating operation of a countersection upon CDS driving;

FIG. 25 is a diagrammatic view illustrating a counting operation in alow illuminance state upon CDS driving;

FIG. 26 is a view but illustrating a counting operation in a highilluminance state upon CDS driving;

FIG. 27 is a timing waveform diagram illustrating operation of a countersection upon DDS driving;

FIG. 28 is a diagrammatic view illustrating a counting operation in alow illuminance state upon DDS driving;

FIG. 29 is a view but illustrating a counting operation in a highilluminance state upon DDS driving;

FIG. 30 is a system block diagram showing a general configuration of aCMOS image sensor according to a second embodiment of the presentdisclosure;

FIG. 31 is a circuit diagram showing an existing configuration of acolumn amplification circuit;

FIG. 32 is a diagrammatic view illustrating an operation range of anoutput signal of the column amplification circuit of FIG. 31;

FIG. 33 is a waveform diagram illustrating an output waveform where thecolumn amplification circuit of FIG. 31 is used to carry out DDSdriving;

FIG. 34A is a circuit diagram showing an example of a configuration of acolumn amplification circuit according to a working example 1 of thesecond embodiment and FIG. 34B is a diagrammatic view illustrating anoperation range of the column amplification circuit of FIG. 31;

FIG. 35 is a diagrammatic view illustrating signal waveforms at severalportions of the column amplification circuit of FIG. 34A;

FIG. 36 is a circuit diagram showing an example of a configuration of acolumn amplification circuit according to a working example 2 of thesecond embodiment;

FIG. 37 is a timing waveform diagram illustrating operation of a CMOSimage sensor which includes the column amplification circuit of FIG. 36;

FIG. 38 is a circuit diagram showing an example of a unit pixelaccording to a different pixel example 1;

FIGS. 39A and 39B are circuit diagrams showing different examples of aunit pixel according to a different pixel example 2;

FIG. 40 is a timing waveform diagram illustrating operation of asolid-state image pickup apparatus having the unit pixel according tothe different pixel example 2 and has a global shutter function;

FIG. 41 is a circuit diagram showing an example of a unit pixelaccording to a different pixel example 3;

FIG. 42 is a timing waveform diagram illustrating operation of asolid-state image pickup apparatus having the unit pixel according tothe different pixel example 3 and has a global shutter function;

FIG. 43 is a block diagram showing an example of a configuration of animage pickup apparatus as an electronic apparatus according to thepresent disclosure;

FIG. 44 is a timing waveform diagram illustrating that, in order toremove fixed pattern noise, it is necessary to read out a reset levelafter a signal level is read out;

FIG. 45 is a diagrammatic view illustrating an example of an in-planedistribution of a reset level in a vertical direction;

FIG. 46 is a diagrammatic view illustrating that, in the case where afixed voltage is applied as a reference voltage, it is necessary toexpand an input voltage range of an AD conversion circuit;

FIG. 47 is a diagrammatic view illustrating a reset level with respectto a reference voltage and a signal level in a maximum amplitude statein the case where the signal amplitude is −1 V in the maximum; and

FIG. 48 is a diagrammatic view illustrating that, in the case where thereset level is varied upwardly and downwardly by a temperaturevariation, it is necessary to expand the input voltage range.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, preferred embodiments of the present invention aredescribed in detail with reference to the accompanying drawings. It isto be noted that the description is given in the following order.

1. First Embodiment (example of a column AD conversion circuit)

1-1. System Configuration

1-2. Pixel Configuration

1-3. Noise Removing Process by Correlation Double Sampling

1-4. Related Art

1-5. Configuration on which the First Embodiment is Based

1-6. Characteristics of the First Embodiment

2. Second Embodiment (example of a column amplification circuit)

2-1. System Configuration

2-2. Characteristics of the Second Embodiment

3. Other Pixel Configurations 4. Modifications

5. Electronic Apparatus (example of an image pickup apparatus)

1. First Embodiment 1-1. System Configuration

FIG. 1 shows a general configuration of a solid-state image pickupapparatus according to a first embodiment of the present invention, forexample, a complementary metal oxide semiconductor (CMOS) image sensorwhich is a kind of X-Y address type solid-state image pickup apparatus.The CMOS image sensor is an image sensor produced applying a CMOSprocess or partially using a CMOS process.

Referring to FIG. 1, the CMOS image sensor 10 _(A) according to thepresent embodiment includes a pixel array section 12 having a pluralityof unit pixels 11 disposed two-dimensionally in rows and columns, andperipheral driving and signal processing systems for driving the unitpixels 11 of the pixel array section 12. In the CMOS image sensor 10_(A) shown in FIG. 1, as the peripheral driving and signal processingsystems, for example, a row scanning section 13, a column processingsection 14 _(A), a reference signal generation section 15, a columnscanning section 16, a horizontal outputting line 17 and a timingcontrolling section 18 are provided. The driving and signal processingsystems are integrated on a semiconductor substrate or chip 19 on whichthe pixel array section 12 is provided.

In the system configuration described, the timing controlling section 18generates clock signals used as a reference for operation of the rowscanning section 13, column processing section 14 _(A), reference signalgeneration section 15, column scanning section 16 and so forth andcontrol signals based on a master clock MCK. The clock signals andcontrol signals generated by the timing controlling section 18 aresupplied as driving signals to the row scanning section 13, columnprocessing section 14 _(A), reference signal generation section 15,column scanning section 16 and so forth.

The pixel array section 12 is configured such that a plurality of unitpixels 11, which may be hereinafter referred to sometimes merely as“pixels,” having a photoelectric conversion section for generating andaccumulating photocharge in response to an amount of light receivedthereby are disposed in a row direction and a column direction, that is,two-dimensionally in a matrix. The row direction is a direction in whichpixels in a pixel row are arrayed, that is, a horizontal direction, andthe column direction is a direction in which pixels of a pixel columnare arrayed, that is, a vertical direction.

In the pixel array section 12, a plurality of row controlling lines 21,that is, 21 ⁻¹ to 21 _(−n), are wired in the row direction for theindividual pixel rows and a plurality of column signal lines 22, thatis, 22 ⁻¹ to 22 _(−m), are wired in the column direction on the array ofthe pixels in the matrix. The row controlling lines 21 transmit acontrol signal for controlling the unit pixels 11 upon reading out fromthe unit pixels 11. While, in FIG. 1, one wire is shown for each of therow controlling lines 21, the number of wires of each row controllinglines 21 is not limited to one. The row controlling lines 21 ⁻¹ to 21_(−n), are connected at one end thereof to output terminals of the rowscanning section 13 which individually correspond to the rows.

The row scanning section 13 is configured from a shift register, anaddress decoder or the like and drives the unit pixels 11 of the pixelarray section 12 simultaneously at a time or in a unit of a row or thelike. In other words, the row scanning section 13 configures a drivingsection for driving the unit pixels 11 of the pixel array section 12together with the timing controlling section 18 which controls the rowscanning section 13. Although a particular configuration of the rowscanning section 13 is not shown, the row scanning section 13 generallyhas two scanning systems including a reading out scanning system and asweeping out scanning system.

The reading out scanning system selectively and successively scans theunit pixels 11 of the pixel array section 12 in a unit of a row in orderto read out signals from the unit pixels 11. The signals read out fromthe unit pixels 11 are analog signals. The sweeping out scanning systemcarries out sweeping scanning preceding by a period of time of a shutterspeed to reading out scanning for a reading out row for which thereading out scanning is to be carried out by the reading out scanningsystem.

By the sweeping out scanning by the sweeping out scanning system,unnecessary charge is swept out from the photoelectric conversionportion of the unit pixels 11 of the reading out row thereby to resetthe photoelectric conversion portions. Then, by sweeping out unnecessarycharge by means of the sweeping out scanning system, that is, byresetting the photoelectric conversion sections, electronic shutteroperation is carried out. The electronic shutter operation is anoperation of discharging photocharge of the photoelectric conversionsections to newly start exposure to light, that is, an operation ofstarting accumulation of photocharge.

Signals read out by the reading out operation by the reading outscanning system correspond to amounts of light received after theimmediately preceding reading out operation or electronic shutteroperation. Then, a period after the reading out timing by theimmediately preceding reading out operation or the sweeping out timingby the electronic shutter operation till the reading out timing by thereading out operation in the current cycle becomes an exposure period ofphotocharge to the unit pixels 11.

The column processing section 14 _(A) includes AD (analog-digital)conversion circuits 23, that is, 23 ⁻¹ to 23 _(−n) provided, forexample, in a one-by-one corresponding relationship to the pixel columnsof the pixel array section 12, that is, to the column signal lines 22,that is, 22 ⁻¹ to 22 _(−m). The AD conversion circuits 23, that is, 23⁻¹ to 23 _(−m), convert an analog signal or pixel signal outputted fromthe unit pixels 11 of the pixel array section 12 for each pixel columninto a digital signal.

The reference signal generation section 15 generates a reference signalV_(ref) of a ramp waveform or inclined waveform whose voltage valuevaries stepwise as time passes. The reference signal generation section15 can be configured using, for example, a DAC (digital-analogconversion) circuit. It is to be noted that the reference signalgeneration section 15 is not limited to a circuit configured using a DACcircuit.

The reference signal generation section 15 a reference signal V_(ref) ofa ramp waveform based on a clock CK supplied thereto from the timingcontrolling section 18 under the control of a controlling signal CS₁supplied thereto from the timing controlling section 18. The referencesignal generation section 15 supplies the generated reference signalV_(ref) to the AD conversion circuits 23 ⁻¹ to 23 _(−m) of the referencesignal generation section 15.

All of the AD conversion circuits 23 ⁻¹ to 23 _(−m), have the sameconfiguration. It is to be noted here that the following description isgiven taking the AD conversion circuit 23 _(−m), for the mth column asan example. The AD conversion circuit 23 _(−m), includes a comparator31, an up/down counter (denoted by “U/D CNT” in FIG. 1) 32 serving as acounting section, a transfer switch 33, and a memory device 34.

The comparator 31 receives a signal voltage V_(out) of the column signalline 22 _(−m) corresponding to a pixel signal outputted from each unitpixel 11 in the nth column of the pixel array section 12 as a comparisoninput thereto. The comparator 31 further receives a reference signalV_(ref) of a ramp wave supplied thereto from the reference signalgeneration section 15 as a reference input thereto. The comparator 31compares the signal voltage V_(out) and the reference signal V_(ref)inputted thereto with each other. Then, the comparator 31 outputs, forexample, an output V_(co) of a first state such as, for example, a highlevel when the reference signal V_(ref) is higher than the signalvoltage V_(out) but outputs the output V_(co) of a second state such as,for example, a low level when the reference signal V_(ref) is equal toor lower than the signal voltage V_(out).

The up/down counter 32 is an asynchronous counter and is supplied withthe clock CK from the timing controlling section 18 at a timing same asthat at the reference signal generation section 15 under the control ofa controlling signal CS₂ supplied thereto from the timing controllingsection 18. The up/down counter 32 carries out down counting or upcounting in synchronism with the clock CK to measure a comparison periodfrom a start to an end of a comparison operation by the comparator 31.

The transfer switch 33 is placed into an on or closed state at a pointof time at which a counting operation of the up/down counter 32regarding the unit pixels 11 in a certain pixel row is completed underthe control of a controlling signal CS₃ supplied thereto from the timingcontrolling section 18. Then, the transfer switch 33 transfers a resultof the counting of the up/down counter 32 to the memory device 34.

In this manner, a comparison operation for analog signals supplied foreach pixel column from each of the unit pixels 11 of the pixel arraysection 12 through the column signal lines 22 ⁻¹ to 22 _(−m), is carriedout by the comparators 31 at first in the AD conversion circuits 23 ⁻¹to 23 _(−m). Then, the up/down counter 32 carries out a countingoperation for a period from a start to an end of the comparisonoperation by the comparator 31 to convert the analog signals intodigital signals, which are stored into the memory devices 34.

The column scanning section 16 is configured from a shift register, anaddress decoder or the like and carries out control of the columnaddress or column scanning of the AD conversion circuits 23 ⁻¹ to 23_(−m), of the column processing section 14 _(A). The digital signalsconverted by the AD conversion circuits 23 ⁻¹ to 23 _(−m), aresuccessively read out to the horizontal outputting line 17 under thecontrol of the column scanning section 16 and outputted as picked upimage data through the horizontal outputting line 17.

It is to be noted that, although not particularly shown because there isno direct relation to the present disclosure, also it is possible toprovide circuits and so forth for carrying out various signal processesfor picked up image data outputted through the horizontal outputtingline 17 in addition to the components described above. In this instance,it does not matter whether or not such circuits are provided on thesemiconductor substrate 19 on which the pixel array section 12 isprovided.

The CMOS image sensor 10 _(A) which incorporates the column-parallel ADCof the configuration described above can carry out global exposure ofexecuting starting and ending of exposure at the same timings to allpixels 11 of the pixel array section 12. This global exposure isexecuted by driving by the driving section configured from the rowscanning section 13 and the timing controlling section 18. The globalshutter function which implements the global exposure is a shutteroperation suitable for use with image pickup of an image pickup objectwhich moves at a high speed or with a sensing application which requiressimultaneity of a picked up image.

It is to be noted that, while, in the present example, the configurationof the column processing section 14 _(A) wherein the AD conversioncircuits 23 are provided in a one-by-one corresponding relationship tothe column signal lines 22 is taken as an example, the column processingsection 14 _(A) is not limited to the configuration wherein the ADconversion circuits 23 and the column signal lines 22 are provided in aone-by-one corresponding relationship to each other. For example, it ispossible to adopt a configuration wherein one AD conversion circuit 23is used commonly by a plurality of pixel columns and is usedtime-divisionally by the plural pixel columns.

1-2. Pixel Configuration

FIG. 2 is a circuit diagram showing an example of a configuration of aunit pixel 11. Referring to FIG. 2, the unit pixel 11 of the presentconfiguration example includes, for example, a photodiode 41 as aphotoelectric conversion section thereof. The unit pixel 11 includes, inaddition to the photodiode 41, for example, a charge detection section42, a transfer transistor or transfer gate section 43, a resettransistor 44, an amplification transistor 45, and a selectiontransistor 46.

It is to be noted here that, for example, an N-channel type MOStransistor is used for the transfer transistor 43, reset transistor 44,amplification transistor 45 and selection transistor 46. However, thecombination of conduction types of the transfer transistor 43, resettransistor 44, amplification transistor 45 and selection transistor 46is a mere example, and the combination is not limited to this.

To the unit pixels 11 of the same pixel row, a plurality of controllines are wired commonly as the row controlling lines 21, that is, 21 ⁻¹to 21 _(−n) described hereinabove. In FIG. 2, the plural control linesare not shown for the simplified illustration. The plural control linesare connected to output terminals of the row scanning section 13corresponding to the pixel rows in a unit of a pixel row. The rowscanning section 13 suitably outputs a transfer signal TRG, a resetsignal RST and a selection signal SEL to the plural control lines.

The photodiode 41 is connected at the anode electrode thereof to anegative side power supply such as, for example, the ground, andphotoelectrically converts light received thereby into photocharge,here, photoelectrons, of a charge amount corresponding to the receivedlight amount and accumulates the photocharge. The photodiode 41 iselectrically connected at the cathode electrode thereof to the gateelectrode of the amplification transistor 45 through the transfertransistor 43.

A region electrically connected to the gate electrode of theamplification transistor 45 is a charge detection section 42 forconverting charge into a voltage. The charge detection section 42 ishereinafter referred to as FD (floating diffusion/floating diffusionregion/impurity diffusion region) section 42.

The transfer transistor 43 is connected between the cathode electrode ofthe photodiode 41 and the FD section 42. To the gate electrode of thetransfer transistor 43, a transfer signal TRG whose high level such asfor example, a V_(dd) level, is an active level (hereinafter referred toas High active) is applied from the row scanning section 13. Thetransfer transistor 43 is placed into a conductive state in response tothe transfer signal TRG to transfer photocharge photoelectricallyconverted by and accumulated into the photodiode 41 to the FD section42.

The reset transistor 44 is connected at the drain electrode thereof to areset potential V_(r) and at the source electrode thereof to the FDsection 42. To the gate electrode of the reset transistor 44, the Highactive reset signal RST is applied from the row scanning section 13. Thereset transistor 44 is placed into a conducting state in response to thereset signal RST to abandon the charge of the FD section 42 to the resetpotential V_(r) thereby to reset the FD section 42.

The amplification transistor 45 is connected at the gate electrodethereof to the FD section 42 and at the drain electrode thereof to thepixel power supply V_(dd). The amplification transistor 45 serves as aninputting portion of a source follower which is a reading out circuitfor reading out a signal obtained by photoelectric conversion by thephotodiode 41. In particular, the amplification transistor 45 isconnected at the source electrode thereof to a column signal line 22through the selection transistor 46 to cooperate with a current source24 connected to one end of the column signal line 22 to configure asource follower.

The selection transistor 46 is connected, for example, at the drainelectrode thereof to the source electrode of the amplificationtransistor 45 and at the source electrode thereof to the column signalline 22. To the gate electrode of the selection transistor 46, the Highactive selection signal SEL is applied from the row scanning section 13.The selection transistor 46 is placed into a conducting state inresponse to the selection signal SEL to place the unit pixel 11 into aselected state thereby to transmit a signal outputted from theamplification transistor 45 to the column signal line 22.

It is to be noted that also it is possible to adopt another circuitconfiguration wherein the selection transistor 46 is connected betweenthe pixel power supply V_(dd) and the drain electrode of theamplification transistor 45.

1-3. Noise Removing Process by Correlation Double Sampling

In the solid-state image pickup apparatus configured such that aplurality of such pixels 11 having the configuration described above aredisposed two-dimensionally in rows and columns, usually a noise removingprocess by correlation double sampling is carried out in order to removenoise upon a resetting operation. Referring to FIG. 3, a unit pixel 11in a state selected by a selection signal SEL resets the FD section 42to a reset potential V_(r) in response to the reset signal RST and readsout the reset potential V_(r) as a reset level V_(rst). Then, thetransfer transistor 43 is driven by the transfer signal TRG to transfercharge accumulated in the photodiode 41 to the FD section 42, and thecharge is read out as a signal level V_(sig).

To the reset level V_(rst) and the signal level V_(sig), noise, that is,random noise, which is generated at random upon each resetting such asthermal noise or noise by coupling by parasitic capacitance is appliedwhen the FD section 42 is reset to the reset potential V_(r). Adifferent noise component is applied every time the FD section 42 isreset.

According to the reading out method by which the reset level V_(rst) isread out first, since random noise generated upon resetting is held bythe FD section 42, a noise amount equal to the reset level V_(rst) isheld in the signal level V_(sig) read out with the signal charge addedthereto. Therefore, a signal from which such noise is removed can beobtained by carrying out the correlation double sampling operation ofsubtracting the reset level V_(rst) from the signal level V_(sig).

In particular, in the correlation double sampling operation, to resetthe FD section 42 and read out the reset level before signal charge istransferred to the FD section 42 is a condition for making it possibleto remove the reset noise. Also noise (fixed pattern noise) which isapplied fixedly such as threshold value noise of the amplificationtransistor 45 used for reading out a signal can be removed.

The reset level V_(rst) and the signal level V_(sig) read out from theunit pixel 11 are converted into digital signals by the AD conversioncircuit 23. Since the range of an input signal which can be converted bythe AD conversion circuit 23 is usually limited, it is necessary to usesuch a design that an analog signal to be outputted from the unit pixel11 is included in the range of the input voltage range of the ADconversion circuit 23.

In other words, the voltage range from the reset level V_(rst) to thesignal level V_(sig) to be acquired has to be included in the inputvoltage range of the AD conversion circuit 23. For example, if the resetlevel V_(rst) is 3 V and the amplitude of the output originating fromthe signal charge is −1 V, then the maximum signal level to be acquiredis 2 V. In this instance, the input voltage range of the AD conversioncircuit 23 for conversion may be 2 V to 3 V.

However, the reset level V_(rst) is actually different among differentpixels due to a threshold dispersion of the amplification transistor 45,an offset by parasitic capacitance with a column signal line 22 and soforth. For example, even if the average value of the reset level V_(rst)of the unit pixels 11 disposed two-dimensionally is 3 V, there is thepossibility that some unit pixel 11 may output a reset level V_(rst) of3.1 V or 2.9 V. In this instance, even if the width of the amplitude bysignal charge is −1 V uniformly, the AD conversion circuit 23 has to beable to convert an input voltage of 1.9 V to 3.1 V. To expand theconvertible input voltage range of the AD conversion circuit 23 isnothing but to enhance a basic performance of the AD conversion circuit23, and this makes a factor of increase of the power supply voltage,power consumption, area and so forth.

From this, preferably the width of the convertible input voltage rangeof the AD conversion circuit 23 is set near to the amplitude of theoutput originating from the signal voltage. Therefore, a method ofadjusting the reference voltage for the input voltage range of the ADconversion circuit 23 is adopted. By shifting the input voltage range ofthe AD conversion circuit 23 with the reference voltage, it is possibleto remove an offset component of an output signal of a pixel, that is,an input signal to the AD conversion circuit 23 to prevent expansion ofthe necessary input voltage range by a dispersion of the reset levelV_(rst).

1-4. Related Art

In the related art disclosed in Patent Document 1, a reset level V_(rst)read out first from a unit pixel 11 is set as a reference voltage V_(zr)of an AD conversion circuit 23 and the reset level V_(rst) and a signallevel V_(sig) are AD converted as seen in FIG. 4. By making thereference voltage V_(zr) equal to the reset level V_(rst) in thismanner, even if the reset level V_(rst) is fluctuated by noise, it ispossible to include an output range |V_(sig)−V_(rst)| of the unit pixel11 by signal charge stably within an input voltage range of the ADconversion circuit 23.

As described hereinabove, generally the reference voltage V_(zr) of theAD conversion circuit 23 can be adjusted only within a sufficientlysmall range with respect to the input voltage range of the AD conversioncircuit 23. Accordingly, the reference voltage V_(zr) of the ADconversion circuit 23 preferably is a signal with regard to which thewidth of a dispersion is stable and limited like the reset levelV_(rst). Conversely, a signal whose voltage varies by a great amount inresponse to an incident light amount like the signal level V_(sig) isnot suitable as the reference voltage V_(zr) of the AD conversioncircuit 23.

In this manner, in the related art disclosed in Patent Document 1, it isassumed that, upon a noise removing process, the reset level V_(rst) isread out first from the unit pixel 11, and the signal level V_(sig) isread out immediately after the reset level V_(rst).

1-5. Configuration on which the First Embodiment is Based

In contrast, the CMOS image sensor 10 _(A) according to the firstembodiment adopts a configuration that signal reading out is executed ina state in which signal charge is held in the FD section 42 for allpixel simultaneous exposure. In other words, the CMOS image sensor 10_(A) according to the first embodiment is configured such that the resetlevel V_(rst) cannot be read out immediately prior to the signal levelV_(sig). Accordingly, under the idea described above, the referencevoltage V_(zr) cannot be acquired from a unit pixel 11 before the signallevel V_(sig) of the same pixel is AD converted.

Meanwhile, in the CMOS image sensor 10 _(A) which implements all pixelsimultaneous exposure, basically an operation of reading out a signalfrom the unit pixels 11 is carried out twice for each one row. As aprerequisite in the case where a reading out operation is carried outtwice, it is assumed that the unit pixels 11 have a pixel structurethat, when the photoelectrically converted charge amount exceeds apredetermined charge amount, signal charge exceeding the predeterminedsaturation charge amount is accumulated into the FD section 42.

It is to be noted that, in the present specification, the illuminancewhen light with which the photoelectrically converted charge amountexceeds a predetermined charge amount is inputted is referred to as“high illuminance.” On the other hand, the illuminance when light withwhich the photoelectrically converted charge amount is equal to orsmaller than the predetermined charge amount is inputted is referred toas “low illuminance.”

In the first time reading out operation of the two times of readingoperation, driving, that is, first driving, of reading out signal chargeretained or accumulated in the FD section 42 as a signal level and thenresetting the FD section 42 to a predetermined potential and reading outthe predetermined potential as a reset level is carried out. In thefollowing description, the first time driving is referred to as “DDS(Double Data Sampling) driving.” In this DDS driving, a signal levelread out first from a unit pixel 11 is used as a reference voltage for aconvertible input voltage range of the AD conversion circuit 23.

In the second time reading out operation, after the FD section 42 isreset to the predetermined potential and the predetermined potential isread out as a reset level, driving, that is, second driving, oftransferring signal charge accumulated in the photodiode 41 to the FDsection 42 and reading out the signal charge of the FD section 42 as asignal level is carried out. In the following description, the secondtime driving is referred to as “CDS (Correlated Double Sampling)driving.” In this CDS driving, a reset level read out first from theunit pixel 11 is used as a reference voltage for a convertible inputvoltage range of the AD conversion circuit 23.

Here, a reason why both of the DDS driving and the CDS driving are usedis described. As apparent from the foregoing description, by using theFD section 42 as a signal retaining or storage section, the dynamicrange of a signal can be expanded in comparison with that in analternate case in which the FD section 42 is not used. However, sincethe DDS driving involves a resetting operation interposed betweenreading out of the signal level and reading out of the reset level, thecorrelation between the reset level included in the signal level readout first and the reset level included in the signal level read outlater is low.

In contrast, in the CDS driving, since the reset level and the signallevel are read out successively after a reset operation is carried out,the correlation between the reset level included in the signal levelread out first and the reset level included in the signal level read outlater is high. Accordingly, since the CDS driving can carry out noiseremoval with a high degree of accuracy in comparison with the DDSdriving, it is advantageous to achieve high picture quality. From thisreason, in order to achieve high picture quality while global exposureis implemented, the driving method which uses both of the DDS drivingand the CDS driving is more preferable than the driving method whichuses only the DDS driving.

Incidentally, in the DDS driving, the signal level read out first fromthe unit pixel 11 is used as a reference voltage for the AD conversioncircuit 23. However, as described hereinabove, the level of the signalwhose voltage varies by a great amount depending upon the incident lightamount originally is not suitable as a reference voltage for the ADconversion circuit 23. In the case where the signal level is used as areference voltage for the AD conversion circuit 23, it is necessary toeliminate the problem of assurance of an operation range of thecomparator 31 which configures the input stage of the AD conversioncircuit 23. This problem is described in detail. However, prior to thedescription, a related configuration of the comparator 31 whichconfigures the input stage of the AD conversion circuit 23 is described.

Related Configuration of a Comparator which Configures the Input Stageof an AD Conversion Circuit

FIGS. 5A and 5B are views showing a related configuration of acomparator which configures the input stage of an AD conversion circuit,and particularly FIG. 5A shows an example of a configuration of acomparator and FIG. 5B illustrates an operation range of the comparator.

As seen in FIG. 5A, the comparator 31 having a related configurationincludes differential pair transistors or comparator transistors 51 and52 having the source electrodes connected commonly, and a current source53 connected between the source common node of the differential pairtransistors 51 and 52 and the ground. The differential pair transistors51 and 52 are formed using an N-channel MOS transistor (hereinafterreferred to as “NMOS transistor”).

To the gate electrode of the NMOS transistor 51, a reference signalV_(refi) of a ramp or stepped waveform generated by the reference signalgeneration section 15 is applied through a capacitor 54. To the gateelectrode of the NMOS transistor 52, a signal voltage V_(out) suppliedfrom a unit pixel 11 through a column signal line 22 _(−m) is appliedthrough a capacitor 55.

A P-channel MOS transistor (hereinafter referred to as “PMOStransistor”) 56 of a diode connection configuration, that is, having thegate electrode and the drain electrode thereof connected commonly, isconnected between the drain electrode of the NMOS transistor 51 and apixel power supply V_(dd). A PMOS transistor 57 is connected between thedrain electrode of the NMOS transistor 52 and the pixel power supplyV_(dd). The PMOS transistors 56 and 57 are connected commonly at thegate electrode thereof.

A PMOS transistor 58 is connected between the gate electrode and thedrain electrode of the NMOS transistor 51. A PMOS transistor 59 isconnected also between the gate electrode and the drain electrode of theNMOS transistor 52. Then, an inverted signal xΦ_(R) of a control signalΦ_(R) for being used for control to set a reference voltage is appliedto the gate of the PMOS transistors 58 and 59.

In the comparator 31 having the configuration described above, areference voltage for the AD conversion circuit 23 is retained into thecapacitors 54 and 55 by application of the inverted signal xΦ_(R) of thecontrol signal Φ_(R) to the gate electrode of the PMOS transistors 58and 59. Then, the comparator 31 compares the reference signal V_(ref)and the signal voltage V_(out) of the column signal line 22 _(−m) witheach other. In particular, the comparator 31 retains a timing at which acomparison result or output V_(co) thereof with the signal voltageV_(out) in response to a change of the reference signal V_(ref) as adigital signal.

The comparison output V_(co) changes when the amplitudes of thereference signal V_(ref) and the signal voltage V_(out) become equal tothat of the reference voltage acquired with the inverted signal xΦ_(R)of the control signal Φ_(R). Referring to FIG. 5B, reference characterV_(th) represents the threshold voltage of a MOS transistor, and ΔVrepresents a potential difference between the reset level and the signallevel at the unit pixel 11. It is to be noted that the AD conversioncircuit 23 to which the present disclosure is applied need notnecessarily have the configuration just described only if it has asetting section for a reference voltage to be used for adjustment of theinput voltage range.

FIG. 6 illustrates a voltage at an internal node of the comparator 31and an operation range at the time of the CDS driving. Meanwhile, FIGS.7 and 8 illustrate a voltage at the internal node of the comparator 31and an operation range at the time when the illuminance upon the DDSdriving is low and high, respectively. As seen in FIGS. 6 to 8, thevoltage within the period within which the reference voltage is set,that is, within a period denoted by AZ in FIGS. 6 to 8, is a voltagewhen the input and the output voltages of the comparator 31 of FIG. 5are short-circuited by the PMOS transistors 58 and 59.

Referring to FIGS. 6 to 8, a mark ∘ indicates a point at which the twoinputs to the comparator 31 coincide with each other. The two inputs arethe voltage V_(outi) on the gate electrode side of the NMOS transistor52 which is displaced by the signal voltage V_(out) of the column signalline 22 _(−m) in response to a pixel signal and the voltage V_(refi) onthe gate electrode side of the NMOS transistor 51 which is displaced bythe reference signal V_(ref) of a ramp waveform supplied from thereference signal generation section 15. This similarly applies also tothose figures which illustrate an operation range.

In the case of the CDS driving illustrated in FIG. 6, the referencesignal V_(ref) is included in an operation range of the comparator 31.In contrast, in the case of the DDS driving illustrated in FIGS. 7 and8, since the reset level is higher than the signal level, it isnecessary for the reference signal V_(ref) for comparison to be higherthan that in the case where the signal level is read. At this time, thevoltage of the reference signal V_(ref) inputted to the comparator 31exceeds the operation range of the comparator 31, the comparator 31 doesnot operate.

That the reference signal V_(ref) exceeds the operation range of thecomparator 31 similarly applies also if the ramp of the reference signalV_(ref) is inverted as seen in FIGS. 9A and 9B. Accordingly, the problemof assurance of the operation range of the comparator 31 cannot besolved fundamentally. It is to be noted that FIG. 9A illustrateswaveforms when the illuminance is low and FIG. 9B illustrates waveformswhen the luminance is high.

Further, also it is imaginable to use a method which adopts a circuitconfiguration wherein the NMOS transistors and the PMOS transistors ofthe comparator 31 are exchanged as seen in FIG. 10A. FIG. 10Aillustrates an example of a configuration of a comparator wherein PMOStransistors are used for the input while FIG. 10B illustrates anoperation range of the comparator of FIG. 10A.

However, the circuit configuration of FIG. 10A cannot assure anoperation range of the CDS driving and cannot carry out CDS driving.Also it is imaginable to prepare both of two kinds of circuits for thecomparator 31, that is, a circuit of the circuit configuration shown inFIG. 5A and another circuit of the circuit configuration shown in FIG.10A and selectively use them for the DDS driving and the CDS driving.Since this method doubles the circuit scale through the use for the CDSdriving, it gives rise to increase of the cost and a problem of adispersion in characteristic because two different circuits are used.

1-6. Characteristics of the First Embodiment

Therefore, in the first embodiment, the CMOS image sensor 10 _(A)including the AD conversion circuit 23 having the comparator 31 forcomparing an analog signal obtained from a unit pixel 11 and thereference signal V_(ref) of a ramp waveform with each other ischaracterized in the following configuration.

In particular, when a signal of a unit pixel 11 is read out divisionallyby two times as a first signal and a second signal, the first signalread out first is used as a reference voltage for an input voltage rangewithin which the input voltage can be processed by a signal processingsection which processes the first and second signals. In the case of thepresent embodiment, the AD conversion circuit 23 corresponds to thesignal processing section which processes the first and second signals.Meanwhile, the reference voltage is a voltage to make a reference withinan input voltage range within which the input voltage can be convertedby the AD conversion circuit 23. In the present embodiment, theoperation range of the comparator 31 is the input voltage range of theAD conversion circuit 23.

At this time, in the case where the first signal is based on signalcharge accumulated or retained in the FD section 42, that is, is thesignal level V_(sig), the signal level V_(sig) is used as the referencevoltage. On the other hand, if the first signal is based on the resetpotential V_(r) when the FD section 42 is reset, that is, is the resetlevel V_(rst), then the reset level V_(rst) is used as the referencevoltage. Then, while the first signal read out first is used as thereference voltage for the AD conversion circuit 23, the AD conversionprocess is carried out for the first and second signals in a state inwhich the internal reference voltage of the AD conversion circuit 23,that is, the initial value for the gate voltages V_(outi) and V_(refi)of the differential pair transistors 51 and 52, is adjusted.

By adjusting the first and second signals such that they may be includedin the input voltage range of the AD conversion circuit 23 in thismanner, the AD conversion circuit 23 can execute the AD conversionprocess for the first and second signals in the state in which the firstand second signals are included in the input voltage range. Accordingly,in whichever the first signal read out first is the signal level V_(sig)or the reset level V_(rst), after a reference voltage is set using asignal of a pixel of a processing target, an AD conversion process canbe carried out with certainty for the first and second signals.

Consequently, such a great difference in fluctuation of a characteristicin a plane, that is, in in-plane distribution or an offset componentwhich relies upon the magnitude of parasitic capacitance as in therelated art disclosed in Patent Document 5, that is, in the case where areference voltage is set using a predetermined voltage generatedexternally and separately, can be removed efficiently. As a result, itis possible to reduce a margin necessary for a convertible input voltagerange of the AD conversion circuit 23 with respect to the outputamplitude of a pixel. Since the margin necessary for the input voltagerange can be reduced, it is possible to lower the power supply voltageand to reduce the power consumption by the AD conversion circuit 23.

Working Example 1

In the following, several working examples of the present invention aredescribed. As described hereinabove, in the DDS driving, driving forreading out signal charge retained or accumulated in the FD section 42as a signal level V_(sig) is carried out. Then, driving of resetting theFD section 42 to the reset potential V_(r) and reading out the resetpotential V_(r) as a reset level V_(rst) is carried out. Then, thesignal level V_(sig) read out first from the unit pixel 11 is used as areference voltage for a convertible input voltage range of the ADconversion circuit 23. In other words, the internal reference voltagesV_(outi) and V_(refi) of the AD conversion circuit 23 are set using thesignal level V_(sig) read out first.

FIGS. 11A and 11B are views of a comparator according to a workingexample 1 which includes a function of adjusting the internal referencevoltages V_(outi) and V_(refi) of the AD conversion circuit 23. Inparticular, FIG. 11A shows an example of a configuration of thecomparator, and FIG. 11B illustrates an operation range of thecomparator.

Referring first to FIG. 11A, the comparator 31 _(A) according to theworking example 1 includes NMOS transistors 51, 52, 56 and 57, PMOStransistors 58 and 59, a current source 53 and capacitors 54 and 55 aswell as an initial voltage setting circuit 70 _(A) for setting oradjusting the internal reference voltages V_(outi) and V_(refi) of theAD conversion circuit 23.

The initial voltage setting circuit 70 _(A) is configured from NMOStransistors 71 and 72 connected between a line L₁ to which an externallyset initial voltage V_(ext) set externally is applied and the gateelectrodes of the NMOS transistors 51 and 52, respectively. To the gateelectrodes of the NMOS transistors 71 and 72, a control signal Φ_(ext)is applied through a control line L₂. This control signal Φ_(ext) isapplied, for example, from the timing controlling section 18 shown inFIG. 1. In other words, the timing controlling section 18 has a functionas a control section for controlling the initial voltage setting circuit70 _(A).

In the comparator 31 _(A) having the configuration described above, uponCDS driving, an initial voltage set by the PMOS transistors 58 and 59being placed into a conducting state in response to an inverted signalxΦ_(R) of the control signal Φ_(R) is retained as the internal referencevoltages V_(outi) and V_(refi) of the AD conversion circuit 23 by thegate side of the NMOS transistors 51 and 52. The initial voltage uponCDS driving, that is, the reference voltage for the AD conversioncircuit 23, is substantially equal to V_(dd)−|V_(th)+ΔV| where V_(th) isthe threshold voltage of the MOS transistors and ΔV is the potentialdifference between the reset level V_(rst) and the signal level V_(sig)of the unit pixel 11.

Upon DDS driving, the NMOS transistors 71 and 72 are placed into aconducting state in response to the control signal Φ_(ext) so that theexternally set initial voltage V_(ext) is applied to the gate electrodeof the NMOS transistors 51 and 52. With the externally set initialvoltage V_(ext), adjustment of the internal reference voltages V_(outi)and V_(refi) of the AD conversion circuit 23 is carried out such thatthe operation range of the comparator 31 _(A) is included in the rangeof a slope of the reference signal V_(ref) of a ramp waveform and thecomparator 31 can operate within the slope.

Upon DDS driving, by adjusting the internal reference voltages V_(outi)and V_(refi) of the AD conversion circuit 23 so that the signal levelV_(sig) and the reset level V_(rst) are included in the input voltagerange of the AD conversion circuit 23 by an operation of the initialvoltage setting circuit 70 _(A), use of both of the CDS driving and theDDS driving becomes possible. Here, that the signal level V_(sig) andthe reset level V_(rst) is included in the input voltage range of the ADconversion circuit 23 signifies that the operation range of thecomparator 31 _(A) is included in the input voltage range within whichthe input voltage of the AD conversion circuit 23 can be converted.

Further, the initial voltage setting circuit 70 _(A) for setting thereference voltages V_(outi) and V_(refi) can be implemented with a verysimple circuit configuration that two NMOS transistors 71 and 72 areadditionally provided. Accordingly, the individual comparators 31 _(A)require increase only of a very small layout area. In other words, thecompatibility of the CDS driving and the DDS driving can be implementedonly by a little increase of the layout area of the comparator 31 _(A).

Here, operation in the case where DDS driving and CDS driving arecarried out alternately within a reading out period for one row isdescribed. FIGS. 12 and 13 illustrate driving waveforms when theilluminance is low and high, respectively, upon DDS driving in the casewhere DDS driving and CDS driving are carried out alternately. FIGS. 12and 13 illustrate waveforms of the inverted signal xΦ_(R) of controlsignal, control signal Φ_(ext), the internal reference voltages V_(outi)and V_(refi) of the AD conversion circuit 23.

While the reference signal V_(ref) is illustrated such that the voltagethereof with respect to time varies in a direction from the high sidetoward the low side, even if the voltage variation direction withrespect to time is reversed such that the voltage varies from the lowside to the high side as seen in FIG. 9A, if the voltage variationremains within the operation range of the comparator 31 _(A), then theDDS driving in the present embodiment is not restricted. Further, in thecase where the comparator is configured such that the PMOS transistorsare provided at the input thereof as seen in FIG. 10A, the initialvoltage setting circuit 70 _(A) may be configured from PMOS transistors73 and 74 as seen in FIG. 14A.

Now, circuit operation of the comparator 31 _(A) according to theworking example 1 shown in FIG. 11A is described with reference to atiming waveform diagram of FIG. 15 in addition to FIGS. 1, 2 and 5A and5B.

First, the selection transistor 46 is placed into a conducting state inresponse to a selection signal SEL_(i) to carry out selection of theunit pixels 11 in the ith row. It is assumed that, at this time, signalcharge is retained or accumulated in the FD section 42. In order todetect the signal level V_(sig) based on the signal charge of the FDsection 42 within an operation range of the comparator 31 _(A), thecontrol signal Φ_(ext) is placed into an active state, that is, into astate of the high potential. Consequently, the initial voltage settingcircuit 70 _(A) is placed into an operative state.

Since the initial voltage setting circuit 70 _(A) is placed into anoperative state, that is, since the NMOS transistors 71 and 72 areplaced into a conducting state, the externally set initial voltageV_(ext) is applied as the gate voltages V_(outi) and V_(refi) to theNMOS transistors 51 and 52, respectively. Consequently, the potential onthe input side of the comparator 31 _(A) is set lower than a potentialset by the PMOS transistors 58 and 59 so that the signal level V_(sig)can be acquired within the operation range of the comparator 31 _(A).This operation by the initial voltage setting circuit 70 _(A) is anoperation for adjusting the reference voltages V_(outi) and V_(refi) sothat the signal level V_(sig) and the reset level V_(rst) may beincluded in the operation range of the comparator 31 _(A), that is, inthe input voltage range of the AD conversion circuit 23.

At this time, also the initial value of the reference signal V_(ref) ofa ramp waveform generated by the reference signal generation section 15is set or shifted to a low potential corresponding to the input sidepotential of the comparator 31 _(A) under the control of the timingcontrolling section 18. Then, the control signal Φ_(ext) is placed intoan inactive state, that is, into a low potential state, therebycompleting the operation for adjusting the potential on the input sideof the comparator 31 _(A), that is, the reference voltage within theconvertible input voltage range of the AD conversion circuit 23.

Thereafter, the reference signal generation section 15 raises thepotential of the reference signal V_(ref) a little from the initialstate within a range within which a dispersion margin of the circuit canbe taken and starts an operation of lowering the voltage in a slopestate or ramp state under the control of the timing controlling section18. Then, the output V_(co) of the comparator 31 _(A) is reversed whenthe difference between the input voltages of the comparator 31 _(A),that is, the difference between the signal level V_(sig) read out fromthe unit pixel 11 and the reference signal V_(ref), becomes equal tozero.

On the other hand, the up/down counter 32 starts, for example, upcounting at the start of the slope of the reference signal V_(ref) andends the counting in response to the reversal of the output V_(co) ofthe comparator 31 _(A). In other words, the up/down counter 32 up countsthe period of time after the point of time of the start of the slope tothe point of time of the reversal of the output V_(co). By this upcounting operation, the difference of the signal level V_(sig) from thereference voltage of the AD conversion circuit 23 can be acquired.

Then, the ith unit pixel 11 is reset with a reset pulse RST_(i) toinitialize the FD section 42. At this time, if the signal level V_(sig)before the resetting is low, that is, if the pixel 11 emits light with ahigh luminance, then the potential of the FD section 42 after theresetting rises by a great amount. Further, even if also the referencesignal V_(ref) has a signal amount equal to that upon the highestluminance, the reference signal V_(ref) is raised in such a manner as todraw a slope. At this time, the up/down counter 32 carries out downcounting from the start of the slope of the reference signal V_(ref).Then, the up/down counter 32 ends the down counting in response toreversal of the output V_(co) of the comparator 31 _(A).

In this manner, the counting operation of the up/down counter 32 iscarried out such that, for example, an up counting operation is carriedout for the first time and a counting operation in the reversedirection, that is, a down counting operation, is carried out for thesecond time. By this counting operation, the subtraction process of the(first time comparison period)−(second time comparison period) iscarried out automatically in the up/down counter 32. Then, when thereference signal V_(ref) and the signal voltage V_(out) of the columnsignal lines 22 ⁻¹ to 22 _(−m), become equal to each other, the polarityof the output V_(co) of the comparator 31 reverses, and in response tothe reversal of the polarity, the counting operation of the up/downcounter 32 stops. As a result, a count value according to a result ofthe subtraction process of the (first time comparison period)−(secondtime comparison period) is retained in the up/down counter 32.

The (first time comparison period)−(second time comparisonperiod)=(signal level V_(sig-i)+reset level V_(rst))−(reset levelV_(rst))=(net signal level V_(sig)). By the two times of reading outoperation and the subtraction process by the up/down counter 32described above, the reset level V_(rst) which includes a dispersion foreach unit pixel 11 is removed, and consequently, the signal levelV_(sig) in accordance with the incident light amount for each unit pixel11 can be extracted. It is to be noted that this signal level V_(sig) isadjusted with the gradient of the slope of the reset level V_(rst) andthe shift amount of the reset level V_(rst) voltage between the signallevel and the reset level so that a higher dynamic range can be takenwith a limited illuminance.

Then, the CDS driving is entered. In the CDS driving, the input/outputof the comparator 31 _(A) are initialized with the control signal Φ_(R).By this initialization operation, a reference voltage of the ADconversion circuit 23 is acquired based on the reset level V_(rst) readout first from the unit pixel 11 and is retained into the capacitors 54and 55. Then, the reference signal generation section 15 raises thereference signal V_(ref) within the operation range of the comparator 31_(A) to start a slope.

At this time, the period of time from the point of time of the start ofthe slope of the reference signal V_(ref) to the point of time of thereversal of the output V_(co) of the comparator 31 _(A) is down countedby the up/down counter 32. By returning the voltage of the referencesignal V_(ref) to the start position of the slope to place the transfertransistor 43 in the unit pixel 11 into a conducting state, signalcharge accumulated in the photodiode 41 is read out. At this time, thepotential of the column signal line 22 changes in response to the chargeamount of the signal charge read out from the photodiode 41. Thepotential of the column signal line 22 drops as the illuminanceincreases.

The reference signal generation section 15 starts generation of thereference signal V_(ref) of a slope which covers a signal read out fromthe unit pixels 11 within the operation range of the comparator 31 _(A).Then, the period of time from the point of time of the start of theslope of the reference signal V_(ref) to the point of time of thereversal of the output V_(co) of the comparator 31 _(A) is up counted bythe up/down counter 32. By this up counting operation, the differencebetween the reset level V_(rst) of the FD section 42 and the signallevel V_(sig) after transfer of the signal charge from the photodiode 41can be detected.

Here, as regards reading out of a result of counting of the up/downcounter 32 to the outside, counting may be carried out separately in theDDS driving and the CDS driving and the count values may be stored intothe memory device 34 and then outputted to the outside. Or, counting maybe carried out continuously in the DDS driving and the CDS driving and aresult of the counting may be stored into the memory device 34 and thenoutputted to the outside.

Further, it is possible to change the gradient of the slope of thereference signal V_(ref) to change the period of time until the outputV_(co) of the comparator 31 _(A) reverses to apply a gain. At this time,it is possible to adopt a technique of changing the gradient of theslope between the DDS driving the CDS driving such that resulting countvalues may have different weights applied thereto.

On the other hand, as regards a resetting operation at the start of theCDS driving which is usually considered necessary, since the FD section42 which is a charge detection section has just been reset, there is nonecessity to carry out resetting again, and therefore, the resettingoperation is omitted. However, in the case where the disturbance to theFD section 42 such as dark current is high, it is preferable to carryout a resetting operation once again between the DDS driving and the CDSdriving to carry out re-initialization.

Further, also as regards the direction of the slope of the referencesignal V_(ref) generated by the reference signal generation section 15,such a method as to make the gradient directions upon the DDS drivingand the CDS driving opposite to each other may be adopted. However, itis preferable to make the gradient directions of the slope upon the DDSdriving and the CDS driving same as each other so that a characteristicdisplacement of the circuit arising from operation in the case of theCDS driving and in the case of the DDS driving may not occur.

Incidentally, in the comparator 31 _(A), current arising from leakcurrent sometimes flows into the inputting portion (gate of the NMOStransistors 51 and 52) from the MOS transistors 58, 59, 71 and 72 whichare switch elements for carrying out initialization to cause the inputpotential to rise or drop. At this time, if the leak amounts on bothinput sides of the comparator 31 _(A) are equal to each other, thenthere is no problem in terms of the characteristic. However, in anactual device, a balance in leak amounts between the two input sides ofthe comparator 31 _(A) is sometimes lost by a fabrication dispersion orthe like.

This problem can be solved by making the channel length L of the MOStransistors 58, 59, 71 and 72 longer because the leak current of thetransistors can be suppressed. However, if the channel length L is madelonger, then clock feedthrough and charge injection which appear whenthe MOS transistors 58, 59, 71 and 72 enter a non-conducting state maypossibly increase, resulting in deterioration of a characteristic of thecomparator 31 _(A).

The clock feedthrough signifies decrease of the voltage drop by acapacitance ratio between a clock controlled node and a floating node.Meanwhile, the charge injection is jumping of charge immediately belowthe channel of the MOS transistors 58, 59, 71 and 72 into a floatingnode. The amount of the clock feedthrough and the charge injectionincreases in proportion to the area of the MOS transistors 58, 59, 71and 72. Accordingly, if the channel length L of the MOS transistors 58,59, 71 and 72 increases, then also the clock feedthrough and the chargeinjection increase inevitably.

From such a reason as described above, the channel length L of the MOStransistors 58, 59, 71 and 72 cannot be increased simply in order tosolve the problem of rise or drop of the input potential arising fromleak current of the MOS transistors 58, 59, 71 and 72. Thus, acomparator according to a working example 2 described below is made tosolve the problem described above without increasing the channel lengthL.

Working Example 2

FIG. 16 is a circuit diagram showing an example of a configuration of acomparator according to a working example 2 which includes a functionfor adjusting the internal reference voltages V_(outi) and V_(refi) ofthe AD conversion circuit 23.

As apparent from comparison between FIGS. 11 and 16, the comparator 31_(B) according to the working example 2 is different in configurationfrom the comparator 31 _(A) according to the working example 1 in aswitch element for carrying out initialization. In particular, in thecomparator 31 _(A) according to the working example 1, each switchelement for carrying out initialization is configured from a single MOStransistor, that is, from the MOS transistor 58, 59, 71 or 72.

In contrast, in the comparator 31 _(B) according to the working example2, two MOS transistors 58 _(A) and 58 _(B) connected in series are usedin place of the PMOS transistor 58, and two MOS transistors 59 _(A) and59 _(B) connected in series are used in place of the PMOS transistor 59.Similarly, two MOS transistors 71 _(A) and 71 _(B) connected in seriesare used in place of the MOS transistor 71, and two MOS transistors 72_(A) and 72 _(B) connected in series are used in place of the MOStransistor 72.

The two MOS transistors 58 _(A) and 58 _(B) have a channel length Lequal to one half that of the PMOS transistor 58, and the two MOStransistors 59 _(A) and 59 _(B) have a channel length L equal to onehalf that of the PMOS transistor 59. Similarly, the MOS transistors 71_(A) and 71 _(B) have a channel length L equal to one half that of theMOS transistor 71, and the two MOS transistors 72 _(A) and 72 _(B) havea channel length L equal to one half the MOS transistor 72.

It is to be noted here that, while the number of MOS transistorsconnected in series in the foregoing description is two, the number isnot limited to two, but three or more MOS transistors may be connectedin series.

A control signal xΦ_(R) is applied to the gate electrode of the MOStransistors 58 _(A) and 59 _(A) while another control signal xΦ_(Rlk)having a later timing and more inactive (in the present example, of ahigher level) than the control signal xΦ_(R) is applied to the gateelectrode of the MOS transistors 58 _(B) and 59 _(B). Further, a furthercontrol signal Φ_(ext) is applied to the gate electrode of the MOStransistors 71 _(A) and 72 _(A) while a still further control signalΦ_(extlk) having a later timing and more inactive (in the presentexample, of a lower level) than the control signal Φ_(ext) is applied tothe gate electrode of the MOS transistors 71 _(B) and 72 _(B).

FIG. 17 illustrates timing waveforms of the control signals xΦ_(R),xΦ_(Rlk), Φ_(ext) and Φ_(extlk) of the comparator 31 _(B) according tothe working example 2, a reference voltages V_(refi), V_(outi).

When the control signal xΦ_(R) is placed into an inactive state and thenthe control signal xΦ_(Rlk) is placed into an inactive state, the MOStransistors 58 _(A) and 59 _(A) on the input side of the comparator 31_(B) are placed into a non-conducting state first, and then the MOStransistors 58 _(B) and 59 _(B) are placed into a non-conducting state.Similarly, when the control signal Φ_(ext) is placed into an inactivestate and then the control signal Φ_(extlk) is placed into an inactivestate, the MOS transistors 71 _(A) and 72 _(A) are placed into anon-conducting state first and then the MOS transistors 71 _(B) and 71_(B) are placed into a non-conducting state.

Working effects provided by the use of a plurality of, for example, two,MOS transistors connected in series as a switch element for carrying outinitialization, that is, working effects of the comparator 31 _(B)according to the working example 2, are described here.

First, working examples in the case where a single MOS transistor isused as a switch element for carrying out initialization, that is, inthe case of the working example 1, are described with reference to FIGS.18A and 18B. It is assumed that a MOS transistor has a channel length Land is in a conducting state (FIG. 18A) and consequently charge Q_(ch)exists in the channel of the MOS transistor. If the MOS transistor isplaced into a non-conducting state (FIG. 18B) from this state, then thecharge Q_(ch) in the channel is divided substantially into two anddistributed to the source and drain regions on the opposite sides of thechannel. Consequently, the injection amount in one of the source anddrain regions decreases to (½)·Q_(ch).

In contrast, in the case where two MOS transistors having a channellength equal to ½ that in the case of the working example 1 areconnected in series, when the two MOS transistors are in a conductingstate (FIG. 18C), an amount of charge equal to ½ in the case where asingle MOS transistor is used, that is, charge of (½)·Q_(ch), exists inthe channel of each of the two MOS transistors.

Then, if the two MOS transistors are successively placed into anon-conducting state (FIG. 18D), then the charge (½)·Q_(ch) in thechannel of one of the two MOS transistors which is placed into anon-conducting state first is divided substantially into two anddistributed to the source and drain regions on the opposite sides of thechannel. Consequently, in the one MOS transistor, the injection amountto one of the source and drain regions becomes (¼)·Q_(ch). Thissimilarly applies also to the other MOS transistors which are placedinto a conducting state later.

In this manner, for example, if the channel length is reduced to ½, thenthe injection amount decreases to ½ that ((½)·Q_(ch)) when the channellength is L, that is, becomes (¼)·Q_(ch). Accordingly, in the case wherethe configuration wherein two MOS transistors having a channel lengthequal to ½ that in the case of the working example 1 are connected inseries is adopted, the injection amount to one of the source/drainregions of the MOS transistors 58, 59, 71 and 72 connected to the twoinput terminals of the comparator 31 _(B) can be reduced to ½. As aresult, the fluctuation of the value of initialization arising from theinjection amount can be suppressed in comparison with that in theworking example 1.

It is to be noted here that, while two MOS transistors having a channellength equal to ½ that in the case of the working example 1 areconnected in series, if there is no problem in the case where theinjection amount is substantially equal to that in the working example1, then also it is possible to adopt a different configuration. Inparticular, the configuration wherein, for example, two MOS transistorshaving a channel length equal to that in the case of the working example1 are connected in series may be adopted. With this configuration, sincethe total channel length of the MOS transistors which configure theswitching element for carrying out initialization can be increased totwice, although the total transistor size increases, the leak currentcan be suppressed in comparison with that in the case of the workingexample 1.

Working Example 3

FIG. 19 is a circuit diagram showing an example of a configuration of acomparator according to a working example 3 which includes a functionfor adjusting the internal reference voltages V_(outi) and V_(refi) ofthe AD conversion circuit 23.

Referring to FIG. 19, the comparator 31 _(C) according to the workingexample 3 has a two-stage circuit structure including a preceding stagecircuit section and a succeeding stage circuit section. The precedingstage circuit section has a circuit configuration same as that of thecomparator 31 _(A) according to the working example 1, that is, has acircuit configuration of a comparison circuit section 81. It is to benoted that the preceding stage circuit section is not limited to that ofa circuit configuration same as that of the comparator 31 _(A) accordingto the working example 1, but may have a circuit configuration same asthat of the comparator 31 _(B) according to the working example 2.

On the other hand, the succeeding stage circuit section is a storagecircuit section 82 which stores an output voltage of the comparisoncircuit section 81 on the preceding stage, that is, a voltage value,that is, a bias voltage, based on or corresponding to the value ofcurrent which depends upon a voltage corresponding to current flowing tothe PMOS transistor 57. The storage circuit section 82 is configured,for example, from a PMOS transistor 821, NMOS transistors 822 and 823,and a capacitor 824.

The PMOS transistor 821 is connected at the gate electrode thereof to anoutput terminal of the comparison circuit section 81, that is, thedrain-common connection node of the MOS transistors 52 and 57 and at thesource electrode thereof to the pixel power supply V_(dd). The NMOStransistor 822 is connected at the drain electrode thereof to the drainelectrode of the PMOS transistor 821 and grounded at the sourceelectrode thereof.

The NMOS transistor 823 is connected at the drain electrode thereof tothe drain electrode of the PMOS transistor 821 and at the sourceelectrode thereof to the gate electrode of the NMOS transistor 822. Thecapacitor 824 is connected at one terminal thereof to the gate electrodeof the NMOS transistor 822 and grounded at the other terminal thereof.Further, the drain-common connection node of the MOS transistors 821 and822 serves as the output terminal of the comparator 31 _(C), and thecomparison output V_(co) is outputted from the drain-common connectionnode.

A control signal xΦ_(R2) is applied to the gate electrode of the NMOStransistor 823. This control signal xΦ_(R2) is supplied, for example,from the timing controlling section 18 shown in FIG. 1. In other words,the timing controlling section 18 has a function as a control sectionfor controlling the succeeding stage circuit section of the comparator31 _(C) according to the working example 3, that is, the storage circuitsection 82.

First Driving Example

Now, a first driving example of the comparator 31 _(C) according to theworking example 3 which includes the storage circuit section 82 on thesucceeding stage of the comparison circuit section 81 is described withreference to a timing waveform diagram of FIG. 20. In FIG. 20, timingwaveforms of the control signals xΦ_(R), xΦ_(ext) and xΦ_(R2), referencevoltages V_(refi), V_(outi) are illustrated.

First, upon CDS driving, in the comparison circuit section 81, aninitial voltage set by placement of the PMOS transistors 58 and 59 intoa conducting state is retained as the internal reference voltagesV_(outi) and V_(refi) into the gate side of the NMOS transistors 51 and52 under the control of the control signal xΦ_(R). In the initializationstate, NMOS transistor 823 of the storage circuit section 82 on thesucceeding stage is placed into a conducting state under the control ofthe control signal xΦ_(R2).

When the NMOS transistor 823 in the storage circuit section 82 on thesucceeding stage is placed into a conducting state, currentcorresponding to the output voltage which depends upon theinitialization value of the comparison circuit section 81 at thepreceding stage flows to the capacitor 824 through the MOS transistors821 and 823. Then, within the initialization period, a voltage valuebased on the current value which depends upon the initial value of theoutput voltage of the comparison circuit section 81 is stored orretained into the capacitor 824 at a timing at which the NMOS transistor823 is placed into a non-conducting state under the control of thecontrol signal xΦ_(R2).

A characteristic of the storage circuit section 82 on the succeedingstage is determined by the current value stored in the capacitor 824.The output of the storage circuit section 82 at the succeeding stage isreversed when the output current of the comparison circuit section 81,that is, the current flowing to the PMOS transistor 821, becomes higherthan the current value generated by the retained voltage of thecapacitor 824.

On the other hand, upon DDS driving, when the NMOS transistors 71 and 72in the comparison circuit section 81 on the preceding stage are placedinto a conducting state under the control of the control signal Φ_(ext),the externally set initial voltage V_(ext) is applied to the gateelectrode of the NMOS transistors 51 and 52. Further, the NMOStransistor 823 in the storage circuit section 82 on the succeeding stageis placed into a conducting state under the control of the controlsignal xΦ_(R2).

When the NMOS transistor 823 in the storage circuit section 82 on thesucceeding stage is placed into a conducting state, currentcorresponding to the output voltage when the input is determined by theexternally set initial voltage V_(ext) flows to the capacitor 824through the MOS transistors 821 and 823. Then, a voltage value based onthe current value which depends upon the externally set initial voltageV_(ext) is stored or retained into the capacitor 824 at a timing atwhich the NMOS transistor 823 is placed into a non-conducting stateunder the control of the control signal xΦ_(R2).

As described hereinabove, in the case of the first driving example, bothupon CDS driving and upon DDS driving, a voltage value based on thecurrent value which depends upon the output voltage of the comparisoncircuit section 81 upon initialization is stored into the storagecircuit section 82. However, as apparent from the description ofoperation given above, a difference appears between the output voltagefor initialization upon CDS driving of the comparison circuit section 81and an output voltage for initialization upon DDS driving of thecomparison circuit section 81.

If a difference appears between the output voltages for initializationupon CDS driving and upon DDS driving in this manner, then since thevoltage value which is stored by the storage circuit section 82, thatis, the voltage value at which the output of the storage circuit section82 is reversed, becomes different upon CDS driving and upon DDS driving.Therefore, the same operating point is not obtained at the comparator 31_(C). In other words, the operating point for initialization of thecomparator 31 _(C) becomes different upon CDS driving and DDS driving. Asecond driving example described below is devised taking this intoconsideration.

Second Driving Example

Subsequently, the second driving example of the comparator 31 _(C)according to the working example 3 is described with reference to atiming waveform diagram of FIG. 21. Also in FIG. 21, the timingwaveforms of control signals xΦ_(R), xΦ_(ext) and xΦ_(R2), referencevoltages V_(refi), V_(outi) are illustrated similarly as in FIG. 20.

The second driving example is same as the first driving example inoperation upon CDS driving but is characterized in operation upon DDSdriving. In particular, as apparent from comparison between the timingwaveform diagram of FIG. 20 and the timing waveform diagram of FIG. 21,upon DDS driving, an operation for initialization same as that upon theCDS driving is carried out before initialization with the externally setinitial voltage V_(ext) is carried out under the control of the controlsignal Φ_(ext).

In particular, prior to initialization with the externally set initialvoltage V_(ext), the comparison circuit section 81 on the precedingstage sets an initial voltage set by placement of the PMOS transistors58 and 59 into a conducting state as the internal reference voltagesV_(outi) and V_(refi) under the control of the control signal xΦ_(R).

In this initialization state, a voltage value corresponding to a currentvalue which depends upon the initial value of the output voltage of thecomparison circuit section 81 is stored or retained into the capacitor824 at a timing at which the NMOS transistor 823 is placed into anon-conducting state under the control of the control signal xΦ_(R2) bythe storage circuit section 82. The voltage value or current valuestored at this time is equal to the voltage value or current valuestored upon initialization upon CDS driving. Consequently, the initialoperating points of the comparator 31 _(C) upon CDS driving and upon DDSdriving, that is, characteristics upon CDS driving and upon DDS driving,become same as each other.

Upon the DDS driving, initialization with the externally set initialvoltage V_(ext) again after initialization same that upon the CDSdriving is carried out to adjust the internal reference voltagesV_(outi) and V_(refi) so that the signal level V_(sig) and the resetlevel V_(rst) may be included in the input voltage range of the ADconversion circuit 23.

As described hereinabove, by carrying out, upon DDS driving,initialization same as that upon CDS driving out prior to initializationwith the externally set initial voltage V_(ext) and then carrying outprimary initialization, that is, initialization with the externally setinitial voltage V_(ext), the problem of the first driving example can besolved. In other words, by carrying out the same initialization bothupon DDS driving and upon CDS driving, characteristics of the operatingpoints of outputs upon DDS driving and upon CDS driving can be made sameas each other. As a result, in addition to the working effects achievedby action of the initial voltage setting circuit 70 _(A), the workingeffect that CDS driving the DDS driving can be carried out compatiblywith a higher degree of certainty can be achieved.

Speeding Up of Driving

Incidentally, in the CMOS image sensor 10 _(A) according to the presentembodiment, CDS driving and DDS driving are carried out simplyalternately to acquire signals. However, naturally a period of timecorresponding to a period of time for CDS driving and DDS driving isrequired for acquisition of signals. In particular, if the driving timeperiod of DDS driving is represented by T_(dds) and the driving timeperiod of CDS driving is represented by T_(cds) as seen in a timingwaveform of FIG. 22, then the time period T_(total) required to acquirea signal by CDS driving and DDS driving is T_(total)=T_(dds)+T_(cds). Inthe case of the present driving example, the slope or inclination of thereference signal V_(ref) of a ramp waveform used upon AD conversion hasthe same gradient both upon CDS driving and DDS driving.

In contrast, if the slope of the reference signal V_(ref) of a rampwaveform for one of the CDS driving and the DDS driving is made steeperthan that for the other one of the CDS driving and the DDS driving, thenthe time required for AD conversion and hence the time before signalsare acquired can be reduced. Data obtained by the driving with regard towhich the slope is made steeper may be multiplied, in the signalprocessing section on the succeeding stage, by a coefficient hereinafterdescribed determined taking a reduction amount of the time, that is, anincreasing amount of the gradient of the slope of the reference signalV_(ref) into consideration. By the calculation, same data by inputconversion can be acquired.

Generally, a signal obtained by DDS driving by an image sensor is inmost cases a signal on the high luminance side. Therefore, the slope ofthe reference signal V_(ref) on the DDS driving side is preferably setto a steeper gradient than that on the CDS driving side.

FIG. 23 illustrates timing waveforms in the case where the slope of thereference signal V_(ref) on the DDS driving side has a steeper gradientA′ than that on the CDS driving side which has the gradient A. Bysetting the slope of the reference signal V_(ref) to the steepergradient A′, the time period required for AD conversion becomesT_(dds)′+T_(cds). Here, reduction of the time period ofT_(dds)′=T_(dds)−(t₁−t₁′)−(t₂−t₂′) is expected. Thus, the time requiredto acquire a signal can be reduced by the reduction time period toachieve speeding up of the driving.

In the case where the inclination or gradient of the slope upon DDSdriving in the case of the driving of FIG. 22 which achieves speedup ofthe driving is represented by A and the inclination of the slope uponDDS driving when speedup is achieved is represented by A′, thecoefficient mentioned hereinabove which is used by the signal processingsection on the succeeding stage is A′/A. In other words, by multiplyingdata obtained by DDS driving where speedup is achieved by thecoefficient of A′/A, output data same as those where speedup is notachieved can be obtained. It is to be noted that the signal process ofthe multiplication by the coefficient may be carried out in a chip orthe semiconductor substrate 19 same as that of the pixel array section12 or outside the chip, and there is no particular restriction in thisregard.

Further, in the case where the all pixels 11 of the pixel array section12 have no signal to be acquired by DDS driving and the signal leveldoes not require the DDS operation itself, for example, in the case ofimage pickup under a very dark environment, it is possible to enhancethe operation speed also by stopping the DDS driving while only the CDSdriving is carried out. In this instance, as an example, such controlmay be imaginable that, if it is detected that an output result withwhich output data upon DDS driving can be determined as a zero outputsuccessively appears for a fixed period of time, then only the CDSdriving is carried out in response to the detection result.

Driving Method for the Counter

As can be recognized from the foregoing description, the AD conversioncircuit 23 of the slope type which uses the reference signal V_(ref) ofa ramp waveform compares the reference signal V_(ref) of a ramp waveformwhich exhibits monotonous decrease or monotonous increase and the signalvoltage V_(out) of a pixel with each other. Then, the period of timeuntil the output of the comparator 31 is reversed is counted with theclock CK, and the count value at the reversal timing of the output ofthe comparator 31 is stored as conversion data to implement ADconversion.

While, in the foregoing description of the embodiment, the up/downcounter 32 is used as the counting section of the column AD conversioncircuit, for example, in the case where the counting section can countwith 12 bits in the positive and negative, a counter which down countsupon CDS driving is incorporated. FIG. 24 illustrates timing waveformsillustrating operation of the counting section upon CDS driving. In theCDS driving, a reversal timing t is down counted till the reset levelV_(rst), and after the counting comes to an end, a complement of thecount value to 1 is calculated to obtain a value whose positive ornegative polarity is set to the reverse −1 with respect to zero.

Then, also with regard to the signal level V_(sig), down counting iscarried out with reference to this value. If it is assumed that thesignal of the pixel 11 has only an equal potential level at a lowilluminance (DARK) and is inputted to the comparator 31, then thecounting ends at the count value −1 as seen in FIG. 25. If the signal ofthe pixel 11 is that of a high illuminance, then counting is carried outdown to −4096 as seen in FIG. 26. Finally, a complement of this data to1 is calculated. Consequently, data of 0 is obtained at the lowilluminance and data of 4095 is obtained at the high illuminance.

However, if it is tried to use the same circuit, that is, the samecounter, to acquire data upon DDS driving, then since, on the highilluminance side, the output of the comparator 31 is reversed aftercounting of a low value, a signal of a level same as that at the lowilluminance upon CDS driving is outputted. In particular, since, at thehigh illuminance upon DDS driving, the output of the comparator 31reverses at a timing proximate to that at the low illuminance upon CDSdriving, but at the low illuminance upon DDS driving, the output of thecomparator 31 reverses at a timing proximate to that at the highilluminance upon CDS driving. Therefore, the compatibility between theinput signal and the data using the same circuit or counter cannot beachieved.

FIG. 27 is a timing waveform diagram illustrating an operation of thecounter section upon DDS driving. Meanwhile, FIG. 28 is a diagrammaticview illustrating a counting operation at the low illuminance upon DDSdriving, and FIG. 29 is a similar view but illustrating a countingoperation at the high illuminance upon DDS driving.

In order to achieve compatibility between an input signal and data usingthe same circuit or counter, in the DDS driving, the counting section ofthe column AD conversion circuit carries out such driving as describedbelow. In particular, signals acquired by the DDS driving are selected,for example, by scanning by the column scanning section 16, and then the4096 data are added and then outputted.

The addition process of the 4096 data may be carried out in the samechip or semiconductor substrate 19 as that of the pixel array section 12or may be carried out as part of a calculation process outside the chip.This addition processing circuit can be implemented by a circuit sectionfor deciding whether or not each signal is a signal acquired by DDSdriving and an adder for receiving results of such decision of thecircuit section and carrying out an addition process upon DDS driving.

In this manner, in the DDS driving, by adding data corresponding to thenumber of bits of the counter, for example, 4096 data in the case of 12bits, to signals acquired by the DDS driving, the compatibility betweenthe input signal and the data using the same circuit or counter can beachieved. In other words, upon acquisition of data by CDS driving andDDS driving, the same circuit or counter can be used as the countingsection of the column AD conversion circuit.

2. Second Embodiment 2-1. System Configuration

FIG. 30 is a system block diagram showing a general configuration of aCMOS image sensor according to a second embodiment of the presentdisclosure.

Referring to FIG. 30, the CMOS image sensor 10 _(B) according to thepresent embodiment includes, in addition to a pixel array section 12,for example, a row scanning section 13, a column processing section 14_(B), a reference signal generation section 15, a column scanningsection 16, a horizontal outputting line 17 and a timing controllingsection 18 as peripheral circuits to the pixel array section 12.

In the CMOS image sensor 10 _(B), only the column processing section 14_(B) is different in configuration from the column processing section 14_(A) in the first embodiment, but the other components, that is, the rowscanning section 13, reference signal generation section 15, columnscanning section 16, horizontal outputting line 17 and timingcontrolling section 18 are basically same as those in the firstembodiment.

The column processing section 14 _(B) has column amplification circuits25, that is, 25 ⁻¹ to 25 _(−m), provided, for example, in a one-by-onecorresponding relationship to pixel columns of the pixel array section12, that is, to column signal lines 22, that is, 22 ⁻¹ to 22 _(−m). Thecolumn amplification circuits 25, that is, 25 ⁻¹ to 25 _(−m), carry outan amplification process for analog signals outputted for the individualcolumns from the unit pixels 11 of the pixel array section 12 and carryout a correlation double sampling process for calculating a differencebetween a signal level and a reset level for the analog signals.

All of the column amplification circuits 25 ⁻¹ to 25 _(−m) have the sameconfiguration. Here, description is given of the column amplificationcircuit 25 _(−m) for the mth column as an example. The columnamplification circuit 25 _(−m) includes an inverting amplifier 61, aninput capacitor 62, a feedback capacitor 63, a control switch 64 and ahorizontal selection switch 65. The column amplification circuit 25_(−m) amplifies a signal voltage V_(out) supplied thereto from a unitpixel 11 through the column signal line 22 _(−m) and outputs theamplified signal voltage V_(out) as an analog signal V_(amp).

It is to be noted that, while the column processing section 14 _(B) inthe present example is configured such that the column amplificationcircuits 25 are provided in a one-by-one corresponding relationship tothe column signal lines 22, the column amplification circuits 25 and thecolumn signal lines 22 need not necessarily be provided in a one-by-onecorresponding relationship. For example, one column amplificationcircuit 25 may be provided commonly to a plurality of pixel columns suchthat it is used time-divisionally by the pixel columns.

Existing Configuration of the Column Amplification Circuit

An existing CMOS image sensor which uses the column amplificationcircuits 25 disposed in parallel in the column direction as a signalprocessing section uses CDS driving as a basic driving method thereof.In particular, the existing CMOS image sensor is based on CDS drivingwherein the reset level V_(rst) is read from a unit pixel 11 first andthen the signal level V_(sig) is read out. Here, an example of aconfiguration of a column amplification circuit used for the CMOS imagesensor according to the existing example is described.

FIG. 31 is a circuit diagram showing an existing configuration of thecolumn amplification circuit 25. FIG. 32 illustrates an operation rangeof an output signal of the column amplification circuit 25 according tothe existing configuration.

Referring first to FIG. 31, the column amplification circuit 25according to the existing configuration includes an inverting amplifier61, an input capacitor 62, a feedback capacitor 63 and a control switch64. Also in this column amplification circuit 25, the input and outputterminals of the inverting amplifier 61 are short-circuited by thecontrol switch 64 under the control of the control signal Φ_(R) toacquire a reference voltage for the column amplification circuit 25.

In the CDS operation, the reset level V_(rst) and the signal levelV_(sig) are read out in this order. However, from the point of view ofthe potential, a high potential and a low potential are read out in thisorder. In the column amplification circuit 25, the output voltage is setlow as an initial value, and in the case where the input signal becomeslow, that is, in the case where the signal level from a unit pixel 11 ishigh, the low initial output voltage is amplified by a capacitance ratiobetween the input capacitor 62 and the feedback capacitor 63. Inparticular, if the capacitance value of the input capacitor 62 isrepresented by C_(in) and the capacitance value of the feedbackcapacitor 63 is represented by C_(fb), the low initial output voltage isamplified by the capacitance ratio C_(in)/C_(fb).

If the initial voltage or reference voltage when the input and outputterminals of the inverting amplifier 61 are short-circuited by thecontrol switch 64 under the control of the control signal Φ_(R), thenthe analog signal V_(amp) of the column amplification circuit 25 isrepresented by

V _(amp) =V _(b)+(C _(in) /C _(fb))ΔV

where ΔV is the potential difference between the reset level V_(rst) andthe signal level V_(sig). The output operation range of the columnamplification circuit 25 is a range from a voltage lower approximatelyby a threshold voltage of the inverting amplifier 61 than the initialvoltage V_(b) when the input and output terminals of the invertingamplifier 61 are short-circuited to an upper operation range whichdepends upon the circuit configuration of the column amplificationcircuit 25.

An output waveform of the column amplification circuit 25 in the casewhere DDS driving is carried out with the CMOS image sensor which usesthe column amplification circuit 25 having the existing configuration isillustrated in FIG. 33.

In the DDS driving, the signal outputting order is reverse to that inthe case of the CDS driving. In other words, the signal level V_(sig)and the reset level V_(rst) are outputted in this order. Accordingly,since the output signal of the column amplification circuit 25 becomesfurther lower than the voltage value of the low initial voltage, theoutput signal goes out of the operation range of the circuit. In orderto solve this, the following configuration is adopted in the secondembodiment.

2-2. Characteristics of the Second Embodiment

In particular, in the second embodiment, the CMOS image sensor 10 _(B)which uses the column amplification circuit 25 which carries out anamplification process for an analog signal from a unit pixel 11 andcarries out a correlation double sampling process of detecting adifference between the signal level V_(sig) and the reset level V_(rst)is characterized in that it adopts the following configuration.

In particular, when a signal of a unit pixel 11 is read out divisionallyby two times as a first signal and a second signal, the first signalread out first is used as a reference voltage for an input voltage rangewhich can be processed by a signal processing section for processing thefirst and second signals. In the case of the present embodiment, thecolumn amplification circuit 25 corresponds to the signal processingsection for processing the first and second signals. Further, thereference voltage is a voltage which is used as a reference to the inputvoltage range within which the column amplification circuit 25 canamplify the voltage.

At this time, in the case where the first signal is a signal based onsignal charge accumulated or retained in the FD section 42, that is, thesignal level V_(sig), then the signal level is used as the referencevoltage. On the other hand, if the first signal is a signal based on thereset potential V_(r) when the FD section 42 is reset, that is, thereset level V_(rst), then the reset level is used as the referencevoltage. Then, while the first signal read out first is used as thereference voltage for the column amplification circuit 25, anamplification process is carried out for the first and second signals ina state in which the reference voltage is adjusted so that the first andsecond signals may be included in the input voltage range of the columnamplification circuit 25.

By adjusting the reference voltage so that the first and second signalsmay be included in the input voltage range of the column amplificationcircuit 25 in this manner, the column amplification circuit 25 can carryout an amplification process for the first and second signals in thestate in which the first and second signals are included in the inputvoltage range. Accordingly, in both of the case in which the firstsignal read out first is the signal level V_(sig) and the case in whichthe first signal is the reset level V_(rst), after a reference voltageis set using a signal of a pixel of a processing object, a signalamplification process can be carried out with certainty to the first andsecond signals.

Consequently, such a great difference in fluctuation of a characteristicwithin a plane or an offset component which relies upon the magnitude ofparasitic capacitance as in the case of the existing technique disclosedin Patent Document 5, that is, as in the case where a predeterminedvoltage separately generated externally is used to set a referencevoltage can be removed effectively. As a result, it is possible toreduce a margin necessary for an input voltage range amplifiable by thecolumn amplification circuit 25 with respect to an output amplitude of apixel. Since a margin necessary for the input voltage range can bereduced, reduction of the power supply voltage to and of the powerconsumption by the column amplification circuit 25 can be anticipated.

Working Example 1

In the following, particular working examples are described. Asdescribed hereinabove, in the DDS driving, driving of reading out signalcharge retained or accumulated in the FD section 42 as a signal levelV_(sig) is carried out. Then, driving of resetting the FD section 42 tothe reset potential V_(r) and reading out the reset potential V_(r) as areset level V_(rst) is carried out. Then, the signal level V_(sig) readout first from the unit pixel 11 is used as a reference voltage for aconvertible input voltage range of the column amplification circuit 25.In other words, the signal level V_(sig) read out first is used to setthe reference voltage for the column amplification circuit 25.

FIGS. 34A and 34B show a column amplification circuit according to aworking example 1 which has a function for adjusting a referencevoltage, and particularly FIG. 34A shows an example of a configurationof the column amplification circuit and FIG. 34B illustrates anoperation range of the column amplification circuit.

Referring first to FIG. 34A, the column amplification circuit 25 _(A)according to the working example 1 includes, in addition to an invertingamplifier 61, an input capacitor 62, a feedback capacitor 63 and acontrol switch 64, an initial voltage setting circuit 70 _(B) forsetting or adjusting the reference voltage of the column amplificationcircuit 25 _(A).

The initial voltage setting circuit 70 _(B) includes an NMOS transistor75 connected in series to the feedback capacitor 63, and a PMOStransistor 76 connected between the connecting node between the feedbackcapacitor 63 and the NMOS transistor 75 and a line L to which anexternally set initial voltage V_(ext) is applied. To the gate electrodeof the NMOS transistor 75 and the PMOS transistor 76, an inverted signalxΦ_(ext) of the control signal Φ_(ext) is applied. The inverted signalxΦ_(ext) of the control signal Φ_(ext) is provided, for example, fromthe timing controlling section 18 shown in FIG. 1. In other words, thetiming controlling section 18 has a function as a control section forcontrolling the initial voltage setting circuit 70 _(B).

FIG. 35 is a waveform diagram illustrating signal waveforms at differentportions of the column amplification circuit 25 _(A) according to theworking example 1. FIG. 35 illustrates the inverted signal xΦ_(ext) ofthe control signal Φ_(ext), control signal Φ_(R), signal voltage V_(out)of the column signal line 22 which is an input voltage to the columnamplification circuit 25 _(A), and analog signal V_(amp) of the columnamplification circuit 25 _(A).

In the case of the CDS driving, the inverted signal xΦ_(ext) of thecontrol signal Φ_(ext) is in a high potential state, and consequently,the NMOS transistor 75 exhibits a conducting state and electricallyconnects the feedback capacitor 63 to the output terminal of the columnamplification circuit 25 _(A). The circuit configuration at this time isthe same as that of the column amplification circuit 25 shown in FIG.31, that is, the same as that upon ordinary CDS driving.

Upon the DDS driving, the input and output terminals of the columnamplification circuit 25 _(A) itself are short-circuited, andconsequently, the input signal of the column amplification circuit 25_(A) becomes equal to the initial voltage or reference voltage V_(b).Consequently, an input signal with which an output amplitude can beassured and the circuit operates is obtained.

On the other hand, when the inverted signal xΦ_(ext) of the controlsignal Φ_(ext) is placed into a low potential state, the NMOS transistor75 is placed into a non-conducting state and the electric connection ofthe feedback capacitor 63 to the output terminal of the columnamplification circuit 25 _(A) is opened. In other words, the feedbackloop of the inverting amplifier 61 is placed into an open state. At thistime, the PMOS transistor 76 is placed into a conducting state andapplies the externally set initial voltage V_(ext) to the open end ofthe feedback capacitor 63. Consequently, the initial value of the outputvoltage of the inverting amplifier 61 becomes equal to the externallyset initial voltage V_(ext). Then, in the case of the DDS driving, sincethe input signal varies from a low signal level to a high reset level,adjustment of the reference voltage of the column amplification circuit25 _(A) is carried out so that the operation range is obtained.

Since the reference voltage upon DDS driving is adjusted by an action ofthe initial voltage setting circuit 70 _(B) so that the signal levelV_(sig) and the reset level V_(rst) may be included in the operationrange of the column amplification circuit 25 _(A) in this manner, use ofboth of the CDS driving and the DDS driving can be achieved.

Further, the initial voltage setting circuit 70 _(B) for adjusting thereference voltage can be implemented by a very simple circuitconfiguration which includes only the two additional MOS transistors 75and 76. Accordingly, each column amplification circuit 25 _(A) exhibitsan increase of a very small area. In other words, use of both of the CDSdriving the DDS driving can be implemented only by an increase by a verysmall area of the column amplification circuit 25 _(A).

Working Example 2

FIG. 36 is a circuit diagram showing an example of a configuration of acolumn amplification circuit according to a working example 2 having afunction for adjusting a reference voltage.

The column amplification circuit 25 _(B) according to the workingexample 2 is configured such that it amplifies a signal at a capacitanceratio between the capacitance on the input side and the capacitance onthe output side. In particular, four output capacitors CN₁, CS₁, CN₂ andCS₂ are connected to the output terminal of an inverting amplifier 61through switches SW₁₁ to SW₁₄, and held charge of the output capacitorsCN₁, CS₁, CN₂ and CS₂ is selectively outputted by switches SW₂₁ to SW₂₄,respectively. The configuration and operation of the other part than thecircuit portion of the capacitance on the output side are same as thosein the case of the column amplification circuit 25 _(A) according to theworking example 1.

At the place of the feedback capacitor 63 of a column amplificationcircuit 25 _(B), an initial voltage setting circuit 70 _(B) for settingor adjusting a reference voltage for the column amplification circuit 25_(B) is provided similarly as in the case of the column amplificationcircuit 25 _(A) according to the working example 1. The initial voltagesetting circuit 70 _(B) has a configuration similar to that of theworking example 1. In particular, the initial voltage setting circuit 70_(B) includes an NMOS transistor 75 connected in series to the feedbackcapacitor 63 and a PMOS transistor 76 for selectively applying anexternally set initial voltage V_(ext) to the connecting node betweenthe feedback capacitor 63 and the NMOS transistor 75.

Now, circuit operation of the column amplification circuit 25 _(B)according to the working example 2 of the configuration described aboveis described with reference to a timing waveform diagram of FIG. 37.

Similarly as in the case of the first embodiment, the externally setinitial voltage V_(ext) as an initial voltage is set so that the outputof the column amplification circuit 25 _(B) upon DDS driving may not besaturated on the lower side. After the setting, an output signal of thecolumn amplification circuit 25 _(B) in a state in which feedback isapplied is sampled into the output capacitor CS₁ with a control signalΦ_(n). Then, the reset signal RST for a pixel 11 is placed into anactive state to sample the signal level into the output capacitor CN₁with a control signal Φ_(N1).

A control signal or reset signal Φ_(R) is inputted to the columnamplification circuit 25 _(B) to reset the FD section 42 of the pixels11 at the same time. Consequently, the input side of the columnamplification circuit 25 _(B) is set to the initial voltage V_(b). Thestate when the application of the control signal Φ_(R) is stopped issampled as an initial value into the output capacitor CN₂ with thecontrol signal Φ_(R2) and then sampled into the output capacitor CS₂with the control signal Φ_(S2) after the signal is transferred from thephotodiode 41 to the FD section 42 with the transfer signal TRG. Asignal level difference upon DDS driving is accumulated in the outputcapacitors CN₁ and CS₁, and a signal level difference upon CDS drivingis accumulated in the output capacitors CN₂ and CS₂. The signals areread out to the output side by horizontal transfer control.

Also in the column amplification circuit 25 _(B) of the configurationwherein a signal is amplified at a capacitance ratio between thecapacitance on the input side and the capacitance on the output side inthis manner, an appropriate initial state can be established by anaction of the initial voltage setting circuit 70 _(B). Consequently, itis made possible to use the same circuit such that an output is includedin the operation range upon both of CDS driving and DDS driving.

Further, the initial voltage setting circuit 70 _(B) for adjusting thereference voltage can be implemented by a very simple circuitconfiguration which includes only the two additional MOS transistors 75and 76. Accordingly, each column amplification circuit 25 _(A) exhibitsan increase of a very small area. In other words, use of both of the CDSdriving and the DDS driving can be implemented only by an increase by avery small area of the column amplification circuit 25 _(B).

Further, by changing the capacitance ratio between the input side andthe feedback side of the column amplification circuit 25 _(B) betweenDDS driving and CDS driving, the signal gain can be set separately forthe DDS driving and the CDS driving. In addition, also it is possible toconnect an AD conversion circuit of the single slope type according tothe first embodiment to the output side of the column amplificationcircuit 25 _(B) such that AD conversion is carried out within a periodrepresented by the control signals Φ_(N1), Φ_(N2), Φ_(S1) and Φ_(S2) toacquire signals.

3. Other Pixel Configurations

In the first and second embodiments, the CMOS image sensors 10 _(A) and10 _(B) having the global exposure function are described as examples ofthe solid-state image pickup apparatus from which a reset level cannotbe read out prior to a signal level. The CMOS image sensors 10 _(A) and10 _(B) having this global exposure function execute a successivereading out operation in a state in which signal charge is held in theFD section 42 as a result of simultaneous transfer of charge generatedin the photodiode 41 to the FD section 42 with regard to the all pixelsin order to implement simultaneous exposure for all pixels.

However, the solid-state image pickup apparatus from which a reset levelcannot be read out prior to a signal level because signal charge to beread out before resetting exists in the signal detection section is notlimited to the CMOS image sensors 10 _(A) and 10 _(B) having the globalexposure function. In the following, some other examples of pixels aredescribed.

Different Pixel Example 1

FIG. 38 is a circuit diagram showing an example of a unit pixelaccording to a different pixel example 1 which uses an organicphotoelectric conversion film.

Referring to FIG. 38, in the unit pixel 11 _(A) according to thedifferent pixel example 1, an organic photoelectric conversion film 281is sandwiched by an upper electrode 282 and a lower electrode 283. Atleast the lower electrode 283 is divided into pixels and is frequentlyformed from an electrode of a high transparency. A bias voltage V_(b) isapplied to the upper electrode 282 from a bias power supply 284.

Charge generated by photoelectric conversion by the organicphotoelectric conversion film 281 is accumulated into the FD section 42.The charge of the FD section 42 is read out as a voltage from the columnsignal line 22 through a reading out circuit including the amplificationtransistor 45. The FD section 42 is set to a drain voltage V_(r) by thereset transistor 44. Then, the drain voltage V_(r) of the resettransistor 44 can be changed from a voltage V_(r1) which is lower thanthe depleted potential of the FD section 42 on the reset transistor 44to another voltage V_(r2) which is higher than the potential.

In the case of the unit pixel 11 _(A) according to the different pixelexample 1 which uses the organic photoelectric conversion film 281 inthis manner, a signal of the pixel is read out by DDS driving. Fromanother pixel which is configured from four transistors and so forth andcan carry out CDS operation and wherein no organic photoelectricconversion film is provided or the function of the organic photoelectricconversion film is disabled, a signal is read out by different driving.For example, in the case where the other pixel which does not use theorganic photoelectric conversion film 281 has, for example, the pixelconfiguration described hereinabove with reference to FIG. 2, readingout by a combination of DDS driving and CDS driving or by only CDSdriving is carried out.

Also to a solid-state image pickup apparatus which uses the unit pixel11 _(A) according to this different pixel example 1, the techniqueregarding the signal processing section according to the first andsecond embodiments described hereinabove can be applied similarly. inother words, both of the DDS driving and the CDS driving can be appliedby the same signal processing section.

Different Pixel Example 2

Now, a unit pixel having a charge accumulation section in addition tothe FD section 42 therein is described as a different pixel example 2.Here, two examples having a pixel configuration which includes a chargeaccumulation section in addition to the FD section 42 are shown in FIGS.39A and 39B.

The unit pixel 11 _(B) according to the pixel configuration shown inFIG. 39A has a pixel configuration wherein an accumulation capacitor 48is connected as a charge accumulation section to the gate electrode ofan amplification transistor 45 through a capacitor selection transistor47. Meanwhile, the unit pixel 11 _(C) according to the pixelconfiguration shown in FIG. 39B has another pixel configuration whereina capacitor selection transistor 47 is connected between anamplification transistor 45 and a reset transistor 44 and anaccumulation capacitor 48 is connected and the ground.

Also to the solid-state image pickup apparatus which uses the unit pixel11 _(B) or 11 _(C) according to the different pixel example 2 of theconfiguration described above, the technique regarding the signalprocessing section according to the first and second embodimentsdescribed hereinabove can be applied similarly. In other words, both ofthe DDS driving and the CDS driving can be carried out by the samesignal processing section.

Here, as an example, an operation in the case where signals are read outfrom a pixel in the order of CDS driving→DDS driving is described as anexample with reference to a timing waveform diagram of FIG. 40.

After the selection transistor 46 is placed into a conducting state witha selection signal SEL, the reset transistor 44 is placed into aconducting state with a reset signal RST to reset the FD section 42. Atthis time, the capacitor selection transistor 47 is kept in anon-conducting state. Then, the potential of the FD section 42 when itis reset is read out as a reset level, and then a transfer signal TRG isplaced into an active state to read out signal charge from thephotodiode 41 to the FD section 42. The series of operations is CDSdriving.

Thereafter, the capacitor selection transistor 47 is placed into aconducting state to read out the signal charge accumulated in the FDsection 42 and the accumulation capacitor 48. Then, the FD section 42and the accumulation capacitor 48 are reset by the reset transistor 44and the reset level is read out. The series of operations is DDSdriving.

Here, while a case in which a reading out operation is carried out inthe order of CDS driving→DDS driving is described as an example, also inthe case where a reading out operation is carried out in the order ofDDS driving→CDS driving, the techniques regarding the signal processingsections according to the first and second embodiments describedhereinabove can be applied similarly.

Different Pixel Example 3

Now, a unit pixel which has an analog memory for signal accumulationtherein and has an intermediate overflow structure is described as adifferent pixel example 3.

FIG. 41 is a circuit diagram showing an example of a unit pixelaccording to the different pixel example 3 which has an analog memoryfor signal accumulation and has an intermediate overflow structure.

The unit pixel 11 _(D) according to the different pixel example 3 isconfigured such that it has an in-pixel accumulation capacitor (MEM) 91which is an analog memory for signal accumulation provided at asucceeding stage of the transfer transistor 43 and includes a transfergate section 92 disposed between the in-pixel accumulation capacitor 91and the FD section 42. The unit pixel 11 _(D) further includes a chargedischarging gate section 40 connected between the photodiode 41 and acharge discharging section (for example, the pixel power supply V_(dd)).If a control signal OFG applied to the gate electrode of the chargedischarging gate section 40 is placed into an active state, then thecharge discharging gate section 40 is placed into a conducting state inresponse to this. Then, the unit pixel 11 _(D) selectively discharges apredetermined amount of photocharge or all photocharge accumulated inthe photodiode 41 from the photodiode 41 to the charge dischargingsection.

In the unit pixel 11 _(C) according to the different pixel example 3,the voltage of the transfer signal TRG to be applied to the transfertransistor 43 is set to an intermediate potential between the highpotential and the low potential. Consequently, if a high illuminancesignal is inputted to the pixel, then charge which becomes equal to orhigher than the potential of the gate of the transfer transistor 43overflows to the capacitor side. Consequently, the charge amount to behandled can be increased.

Operation of a solid-state image pickup apparatus which has the unitpixel 11 _(D) according to the different pixel example 3 and has theglobal shutter function is described with reference to a timing waveformdiagram of FIG. 42.

First, starting of accumulation time is carried out by global resettingto initialize all pixels at a time. After the accumulation time passes,charge accumulated in the in-pixel accumulation capacitor 91 by anoverflow is transferred to the FD section 42 at a time with regard toall pixels under the control of the transfer gate section 92.Thereafter, the transfer transistor 43 is placed into a conducting stateat a time with regard to all pixels to transfer a signal on the lowilluminance side accumulated in the photodiode 41 to the in-pixelaccumulation capacitor 91. Before and after the transfer, signalsoriginally existing in the photodiode 41 and the in-pixel accumulationcapacitor 91 move to the in-pixel accumulation capacitor 91 and the FDsection 42 at a time with regard to all pixels.

As seen in FIG. 42, the signal of the FD section 42 is read out by DDSdriving and the signal of the in-pixel accumulation capacitor 91 is readout by CDS driving. It is to be noted that, since it is assumed that theselection signal SEL upon global exposure is in an inactive state, whenthe reset signal RST upon reading out is inputted, preferably theselection signal SEL is in an inactive state in order that it may have acorrelation to the state of the pixel. Also the control signal Φ_(ext)and the control signal Φ_(R) at this time are inputted at appropriatetimings upon CDS driving and DDS driving described above.

4. Modifications

In the embodiments described hereinabove, the DDS driving and the CDSdriving are carried out basically in the order of the DDS driving→CDSdriving. However, the number of times of reading out in the CDS drivingis not limited to one time. While a switch is sometimes represented by asymbol in order to simplify illustration of the drawings, also it ispossible to use an NMOS or PMOS transistor alternatively. Further, theoperations described hereinabove are possible if the NMOS or PMOStransistor is used such that the High level of the NMOS transistorcorresponds to the active state of the switch and the Low level of thePMOS transistor corresponds to the active state of the switch.

Further, although it is described hereinabove that DDS driving and CDSdriving are executed alternately for each one row within a reading outperiod of one row, the driving method is not limited to this specificdriving method. For example, also it is possible to adopt a differentdriving method wherein DDS driving and CDS driving are executedalternately for each one frame. According to this driving method, sincechangeover between the DDS driving and the CDS driving may be carriedout in a period of one frame, there is an advantage that higher speedoperation can be anticipated in comparison with an alternative case inwhich such changeover is carried out in a period of one line or one row.On the other hand, changeover in a period of one frame requires a framememory in the signal processing section on the succeeding stage.Therefore, from a point of view of the memory capacity, changeover in aperiod of one line is superior.

Further, while, in the embodiments described hereinabove, the presentinvention is applied to a CMOS image sensor wherein unit pixels aredisposed in rows and columns, the application of the disclosure is notlimited to a CMOS image sensor. In particular, the present disclosurecan be applied to general solid-state image pickup apparatus of the X-Yaddress type wherein unit pixels are disposed two-dimensionally in rowsand columns.

Further, the present disclosure can be applied not only to solid-stateimage pickup apparatus which detect and pick up a distribution of theincident light amount of visible rays as an image but can be appliedalso to general solid-state image pickup apparatus which pick up adistribution of an incident amount of infrared rays, X rays, particlesor the like as an image.

It is to be noted that a solid-state image pickup apparatus may have aform in which it is formed as a one-chip device or may have a form of amodule having an image pickup function in which an image pickup sectionand a signal processing section or an optical system are packagedcollectively.

5. Electronic Apparatus

The present disclosure is not limited to applications to a solid-stateimage pickup apparatus but can be applied to general electronicapparatus which use a solid-state image pickup apparatus as an imagecapturing section or a photoelectric conversion section thereof such asan image pickup apparatus of a digital still camera or a video camera, aportable terminal apparatus having an image pickup apparatus such as aportable telephone set and so forth. Electronic apparatus which uses asolid-state image pickup apparatus as an image capturing section thereofincludes a copying machine which uses a solid-state image pickupapparatus as an image reading station thereof. It is to be noted thatthe image pickup apparatus may have a form of a module describedhereinabove which is incorporated in an electronic apparatus, that is,the image pickup apparatus may have a form of a camera module.

Image Pickup Apparatus

FIG. 43 is a block diagram showing an example of a configuration of anelectronic apparatus according to the present disclosure, for example,of an image pickup apparatus.

Referring to FIG. 43, an image pickup apparatus 100 according to thepresent disclosure includes an optical system including a lens group 101and so forth, an image pickup device 102, a DSP circuit 103, a framememory 104, a display apparatus 105, a recording apparatus 106, anoperation system 107, a power supply system 108 and so forth. The DSPcircuit 103, frame memory 104, display apparatus 105, recordingapparatus 106, operation system 107 and power supply system 108 areconnected to each other by a bus line 109.

The lens group 101 fetches incident light or image light from an imagepickup object and forms an image on an image pickup face of the imagepickup device 102. The image pickup device 102 converts the amount ofthe incident light of the image formed on the image pickup face thereofby the lens group 101 into electric signals and outputs the electricsignals as pixel signals.

The display apparatus 105 is formed from a panel type display unit suchas a liquid crystal display unit, an organic EL (electroluminescence)display unit or the like and displays a moving picture or a stillpicture picked up by the image pickup device 102. The recordingapparatus 106 records the moving picture or the still picture picked upby the image pickup device 102 on a recording medium such as a videotape or a DVD (Digital Versatile Disk).

The operation system 107 issues operation instructions regarding variousfunctions which the image pickup apparatus has in response to anoperation thereof by a user. The power supply system 108 suitablysupplies various powers serving as operation power supplies to the DSPcircuit 103, frame memory 104, display apparatus 105, recordingapparatus 106 and operation system 107 to the power supply objects.

The image pickup apparatus of the configuration described above can beused as an image pickup apparatus for a video camera, a digital stillcamera, a camera module for a mobile apparatus such as a portabletelephone set and so forth. Then, if, in the image pickup apparatus, asolid-state image pickup apparatus such as the CMOS image sensor 10_(A), 10 _(B) or the like according to the embodiments describedhereinabove is used as the image pickup device 102, then such workingeffects as described below can be achieved.

In particular, the CMOS image sensors 10 _(A) and 10 _(B) according tothe embodiments described hereinabove can implement pickup of an imagefree from distortion by global exposure. Accordingly, the CMOS imagesensors 10 _(A) and 10 _(B) can be implemented as an image pickupapparatus which can be applied suitably for pickup of an image of animage pickup object which moves at a high speed and which cannot allowimage distortion or for sensing applications which require simultaneityof picked up images.

Further, the CMOS image sensors 10 _(A) and 10 _(B) according to theembodiments described hereinabove can effectively remove a difference ofa great fluctuation of a characteristic in a plane (in-planedistribution) or an offset component which depends upon the magnitude ofparasitic capacitance. Consequently, since it is possible to reduce amargin necessary for an input voltage range amplifiable by the signalprocessing section with respect to an output amplitude of a pixel,reduction in power supply voltage and power consumption of the signalprocessing section can be anticipated. Accordingly, the presentinvention can contribute to reduction in voltage and power consumptionof various electronic apparatus.

The present application is a continuation application of applicationSer. No. 13/064,338, filed Mar. 21, 2011, which in turn contains subjectmatter related to that disclosed in Japanese Priority PatentApplications JP 2011-015994 filed in the Japan Patent Office on Jan. 28,2011 and JP 2010-079017 filed in the Japan Patent Office On Mar. 30,2010, the entire content of which is hereby incorporated by reference.

While preferred embodiments of the present invention have been describedusing specific terms, such description is for illustrative purpose only,and it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

What is claimed is:
 1. An imaging device comprising: a pixel configuredto output a pixel signal; a signal line configured to read out the pixelsignal; and a comparator including: first and second differentialtransistors, a first capacitor disposed between a gate of the firstdifferential transistor and a reference signal generation sectionconfigured to supply a reference signal, a second capacitor disposedbetween a gate of the second differential transistor and the signalline, a first circuit disposed between the gate of the firstdifferential transistor and a predetermined voltage line, and a secondcircuit disposed between the gate of the second differential transistorand the predetermined voltage line.
 2. The imaging device according toclaim 1, wherein the first circuit includes a third transistor and afourth transistor, one of a source or a drain of the third transistor isconnected to the gate of the first differential transistor and the otherof the source or the drain of the third transistor is connected to oneof a source or a drain of the fourth transistor, and the other of thesource or the drain of the fourth transistor is connected to thepredetermined voltage line.
 3. The imaging device according to claim 2,wherein the second circuit includes a fifth transistor and a sixthtransistor, one of a source or a drain of the fifth transistor isconnected to the gate of the second differential transistor and theother of the source or the drain of the fifth transistor is connected toone of a source or a drain of the sixth transistor, and the other of thesource or the drain of the sixth transistor is connected to thepredetermined voltage line.
 4. The imaging device according to claim 1,further comprising: a seventh transistor and an eighth transistor,wherein the seventh transistor is disposed between the firstdifferential transistor and the eighth transistor; and a ninthtransistor and a tenth transistor, wherein the ninth transistor isdisposed between the second differential transistor and the tenthtransistor.
 5. The imaging device according to claim 4, wherein a gateof the seventh transistor is configured to receive a first controlsignal, and a gate of the eighth transistor is configured to receive asecond control signal.
 6. The imaging device according to claim 5,wherein the first control signal is different from the second controlsignal.
 7. The imaging device according to claim 5, wherein a gate ofthe ninth transistor is configured to receive a third control signal,and a gate of the tenth transistor is configured to receive a fourthcontrol signal.
 8. The imaging device according to claim 7, wherein thethird control signal is different from the fourth control signal.
 9. Theimaging device according to claim 7, wherein the first control signal isthe same as the third control signal.
 10. An electronic apparatuscomprising: an optical system; and the imaging device according toclaim
 1. 11. An imaging device comprising: a pixel configured to outputa pixel signal; a signal line configured to read out the pixel signal;and a comparator including: first and second differential transistors, afirst capacitor disposed between a gate of the first differentialtransistor and a reference signal generation section configured tosupply a reference signal, a second capacitor disposed between a gate ofthe second differential transistor and the signal line, a first circuitdisposed between the gate of the first differential transistor and oneof a source or a drain of the first differential transistor, a thirdtransistor disposed between the gate of the first differentialtransistor and a predetermined voltage line, a second circuit disposedbetween the gate of the second differential transistor and one of asource or a drain of the second differential transistor, and a fourthtransistor disposed between the gate of the second differentialtransistor and the predetermined voltage line.
 12. The imaging deviceaccording to claim 11, wherein the first circuit includes a fifthtransistor and a sixth transistor, the fifth transistor being disposedbetween the sixth transistor and the first differential transistor, andthe second circuit includes a seventh transistor and an eighthtransistor, the seventh transistor being disposed between the eighthtransistor and the second differential transistor.
 13. The imagingdevice according to claim 12, wherein a gate of the fifth transistor isconfigured to receive a first control signal and a gate of the sixthtransistor is configured to receive a second control signal.
 14. Theimaging device according to claim 13, wherein the first control signalis different form the second control signal.
 15. The imaging deviceaccording to claim 13, wherein a gate of the seventh transistor isconfigured to receive a third control signal and a gate of the eighthtransistor is configured to receive a fourth control signal.
 16. Theimaging device according to claim 15, wherein the third control signalis different from the fourth control signal.
 17. The imaging deviceaccording to claim 15, wherein the first control signal is the same asthe third control signal.
 18. The imaging device according to claim 11,wherein the first circuit and the second circuit respectively include atleast three transistors.
 19. An electronic apparatus comprising: anoptical system; and the imaging device according to claim 11.